Extend the concept of `INIT-CMDS' to the AC_CONFIG_FOOS.

Extend the concept of `INIT-CMDS' to the AC_CONFIG_FOOS.

* acgeneral.m4 (_AC_CONFIG_COMMANDS_INIT): New macro which will
collect the `INIT-CMDS'.
(AC_CONFIG_COMMANDS, AC_CONFIG_LINKS, AC_CONFIG_HEADERS,
AC_CONFIG_FILES): Use it, and also temporarily divert to -1
instead of spamming dnl everywhere.
(AC_OUTPUT_CONFIG_STATUS): Make sure to output the INIT-CMDS
before the other sections, it'd be a pity to initialize after the
use :).

* doc/autoconf.texi (Configuration Actions): New section,
documenting the common behavior of AC_CONFIG_FILES,
AC_CONFIG_HEADERS, macro AC_CONFIG_COMMANDS, and AC_CONFIG_LINKS.
(Configuration Files): Document $2 and $3 of AC_CONFIG_FILES.
(Configuration Headers): Document $2 and $3 of AC_CONFIG_HEADERS.
(Configuration Commands): Document $2 and $3 of AC_CONFIG_COMMANDS.
(Configuration Links): Document $2 and $3 of AC_CONFIG_FILES.