BUILD: cfgparse: silence a bogus gcc warning on 32-bit machines

BUILD: cfgparse: silence a bogus gcc warning on 32-bit machines

A first patch was made during 2.0-dev to silence a bogus warning emitted
by gcc : dd1c8f1f72 ("MINOR: cfgparse: Add a cast to make gcc happier."),
but it happens it was not sufficient as the warning re-appeared on 32-bit
machines under gcc-8 and gcc-9 :

  src/cfgparse.c: In function 'check_config_validity':
  src/cfgparse.c:3642:33: warning: argument 1 range [2147483648, 4294967295] exceeds maximum object size 2147483647 [-Walloc-size-larger-than=]
       newsrv->idle_orphan_conns = calloc((unsigned int)global.nbthread, sizeof(*newsrv->idle_orphan_conns));
                                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This warning doesn't trigger in other locations, and it immediately
vanishes if the previous or subsequent loops do not depend on
global.nbthread anymore, or if the field ordering of the struct server
changes! As discussed in the thread at:

   https://www.mail-archive.com/haproxy@formilux.org/msg36107.html

playing with -Walloc-size-larger-than has no effect. And a minimal
reproducer could be isolated, indicating it's pointless to circle around
this one. Let's just cast nbthread to ushort so that gcc cannot make
this wrong detection. It's unlikely we'll use more than 65535 threads in
the near future anyway.

This may be backported to older releases if they are also affected, at
least to ease the job of distro maintainers.

Thanks to Ilya for testing.