configure.ac, lib/, src/: Use gid_t instead of GETGROUPS_T

configure.ac, lib/, src/: Use gid_t instead of GETGROUPS_T

Autoconf's NEWS file says

*** AC_FUNC_GETGROUPS and AC_TYPE_GETGROUPS no longer run test programs.
  These macros were testing for OS bugs that we believe are at least
  twenty years in the past.  Most operating systems are now trusted to
  provide an accurate prototype for getgroups in unistd.h, and to
  implement it as specified in POSIX.

Reviewed-by: Serge Hallyn <serge@hallyn.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>