BUG/MINOR: cfgparse: Fix argument reference in PARSE_ERR_TOOMANY message

BUG/MINOR: cfgparse: Fix argument reference in PARSE_ERR_TOOMANY message

The returned `arg` value is the number of arguments found, but in case
of the error message it's not a valid argument index.

Because we know how many arguments we allowed (MAX_LINE_ARGS) we know
what to print in the error message, so do just that.

Consider a configuration like this:

    listen foo
      1 2 3 [...] 64 65

Then running a configuration check within valgrind reports the following:

    ==18265== Conditional jump or move depends on uninitialised value(s)
    ==18265==    at 0x56E8B83: vfprintf (vfprintf.c:1631)
    ==18265==    by 0x57B1895: __vsnprintf_chk (vsnprintf_chk.c:63)
    ==18265==    by 0x4A8642: vsnprintf (stdio2.h:77)
    ==18265==    by 0x4A8642: memvprintf (tools.c:3647)
    ==18265==    by 0x4CB8A4: print_message (log.c:1085)
    ==18265==    by 0x4CE0AC: ha_alert (log.c:1128)
    ==18265==    by 0x459E41: readcfgfile (cfgparse.c:1978)
    ==18265==    by 0x507CB5: init (haproxy.c:2029)
    ==18265==    by 0x4182A2: main (haproxy.c:3137)
    ==18265==
    ==18265== Use of uninitialised value of size 8
    ==18265==    at 0x56E576B: _itoa_word (_itoa.c:179)
    ==18265==    by 0x56E912C: vfprintf (vfprintf.c:1631)
    ==18265==    by 0x57B1895: __vsnprintf_chk (vsnprintf_chk.c:63)
    ==18265==    by 0x4A8642: vsnprintf (stdio2.h:77)
    ==18265==    by 0x4A8642: memvprintf (tools.c:3647)
    ==18265==    by 0x4CB8A4: print_message (log.c:1085)
    ==18265==    by 0x4CE0AC: ha_alert (log.c:1128)
    ==18265==    by 0x459E41: readcfgfile (cfgparse.c:1978)
    ==18265==    by 0x507CB5: init (haproxy.c:2029)
    ==18265==    by 0x4182A2: main (haproxy.c:3137)
    ==18265==
    ==18265== Conditional jump or move depends on uninitialised value(s)
    ==18265==    at 0x56E5775: _itoa_word (_itoa.c:179)
    ==18265==    by 0x56E912C: vfprintf (vfprintf.c:1631)
    ==18265==    by 0x57B1895: __vsnprintf_chk (vsnprintf_chk.c:63)
    ==18265==    by 0x4A8642: vsnprintf (stdio2.h:77)
    ==18265==    by 0x4A8642: memvprintf (tools.c:3647)
    ==18265==    by 0x4CB8A4: print_message (log.c:1085)
    ==18265==    by 0x4CE0AC: ha_alert (log.c:1128)
    ==18265==    by 0x459E41: readcfgfile (cfgparse.c:1978)
    ==18265==    by 0x507CB5: init (haproxy.c:2029)
    ==18265==    by 0x4182A2: main (haproxy.c:3137)
    ==18265==
    ==18265== Conditional jump or move depends on uninitialised value(s)
    ==18265==    at 0x56E91AF: vfprintf (vfprintf.c:1631)
    ==18265==    by 0x57B1895: __vsnprintf_chk (vsnprintf_chk.c:63)
    ==18265==    by 0x4A8642: vsnprintf (stdio2.h:77)
    ==18265==    by 0x4A8642: memvprintf (tools.c:3647)
    ==18265==    by 0x4CB8A4: print_message (log.c:1085)
    ==18265==    by 0x4CE0AC: ha_alert (log.c:1128)
    ==18265==    by 0x459E41: readcfgfile (cfgparse.c:1978)
    ==18265==    by 0x507CB5: init (haproxy.c:2029)
    ==18265==    by 0x4182A2: main (haproxy.c:3137)
    ==18265==
    ==18265== Conditional jump or move depends on uninitialised value(s)
    ==18265==    at 0x56E8C59: vfprintf (vfprintf.c:1631)
    ==18265==    by 0x57B1895: __vsnprintf_chk (vsnprintf_chk.c:63)
    ==18265==    by 0x4A8642: vsnprintf (stdio2.h:77)
    ==18265==    by 0x4A8642: memvprintf (tools.c:3647)
    ==18265==    by 0x4CB8A4: print_message (log.c:1085)
    ==18265==    by 0x4CE0AC: ha_alert (log.c:1128)
    ==18265==    by 0x459E41: readcfgfile (cfgparse.c:1978)
    ==18265==    by 0x507CB5: init (haproxy.c:2029)
    ==18265==    by 0x4182A2: main (haproxy.c:3137)
    ==18265==
    ==18265== Conditional jump or move depends on uninitialised value(s)
    ==18265==    at 0x56E941A: vfprintf (vfprintf.c:1631)
    ==18265==    by 0x57B1895: __vsnprintf_chk (vsnprintf_chk.c:63)
    ==18265==    by 0x4A8642: vsnprintf (stdio2.h:77)
    ==18265==    by 0x4A8642: memvprintf (tools.c:3647)
    ==18265==    by 0x4CB8A4: print_message (log.c:1085)
    ==18265==    by 0x4CE0AC: ha_alert (log.c:1128)
    ==18265==    by 0x459E41: readcfgfile (cfgparse.c:1978)
    ==18265==    by 0x507CB5: init (haproxy.c:2029)
    ==18265==    by 0x4182A2: main (haproxy.c:3137)
    ==18265==
    ==18265== Conditional jump or move depends on uninitialised value(s)
    ==18265==    at 0x56E8CAB: vfprintf (vfprintf.c:1631)
    ==18265==    by 0x57B1895: __vsnprintf_chk (vsnprintf_chk.c:63)
    ==18265==    by 0x4A8642: vsnprintf (stdio2.h:77)
    ==18265==    by 0x4A8642: memvprintf (tools.c:3647)
    ==18265==    by 0x4CB8A4: print_message (log.c:1085)
    ==18265==    by 0x4CE0AC: ha_alert (log.c:1128)
    ==18265==    by 0x459E41: readcfgfile (cfgparse.c:1978)
    ==18265==    by 0x507CB5: init (haproxy.c:2029)
    ==18265==    by 0x4182A2: main (haproxy.c:3137)
    ==18265==
    ==18265== Conditional jump or move depends on uninitialised value(s)
    ==18265==    at 0x56E8CE2: vfprintf (vfprintf.c:1631)
    ==18265==    by 0x57B1895: __vsnprintf_chk (vsnprintf_chk.c:63)
    ==18265==    by 0x4A8642: vsnprintf (stdio2.h:77)
    ==18265==    by 0x4A8642: memvprintf (tools.c:3647)
    ==18265==    by 0x4CB8A4: print_message (log.c:1085)
    ==18265==    by 0x4CE0AC: ha_alert (log.c:1128)
    ==18265==    by 0x459E41: readcfgfile (cfgparse.c:1978)
    ==18265==    by 0x507CB5: init (haproxy.c:2029)
    ==18265==    by 0x4182A2: main (haproxy.c:3137)
    ==18265==
    ==18265== Conditional jump or move depends on uninitialised value(s)
    ==18265==    at 0x56EA2DB: vfprintf (vfprintf.c:1632)
    ==18265==    by 0x57B1895: __vsnprintf_chk (vsnprintf_chk.c:63)
    ==18265==    by 0x4A8642: vsnprintf (stdio2.h:77)
    ==18265==    by 0x4A8642: memvprintf (tools.c:3647)
    ==18265==    by 0x4CB8A4: print_message (log.c:1085)
    ==18265==    by 0x4CE0AC: ha_alert (log.c:1128)
    ==18265==    by 0x459E41: readcfgfile (cfgparse.c:1978)
    ==18265==    by 0x507CB5: init (haproxy.c:2029)
    ==18265==    by 0x4182A2: main (haproxy.c:3137)
    ==18265==
    [ALERT] 174/165720 (18265) : parsing [./config.cfg:2]: too many words, truncating at word 65, position -95900735: <(null)>.
    [ALERT] 174/165720 (18265) : Error(s) found in configuration file : ./config.cfg
    [ALERT] 174/165720 (18265) : Fatal errors found in configuration.

Valgrind reports conditional jumps relying on an undefined value and the
error message clearly shows incorrect stuff.

After this patch is applied the relying on undefined values is gone and
the <(null)> will actually show the argument. However the position value
still is incorrect. This will be fixed in a follow up patch.

This patch fixes up commit 9e1758efbd68c8b1d27e17e2abe4444e110f3ebe which
is 2.2 only. No backport needed.