id: handle systems without getgroups support

id: handle systems without getgroups support

If getgroups failed with ENOSYS, mgetgroups would unnecessarily
fail, and that provoked id into freeing an uninitialized pointer.
Meanwhile, we were not using xalloc_die properly.  Both issues
are better solved in gnulib, by introducing xgetgroups; this
patch uses the new interface.

Regression introduced by commit 6a31fd8d7.

* gnulib: Update, for mgetgroups improvments.
* src/id.c (print_full_info): Adjust caller to die on allocation
failure, and no longer worry about ENOSYS.
* src/group-list.c (print_group_list): Likewise.
* src/setuidgid.c (main): Likewise.
* NEWS: Mention the fix.
* THANKS: Update.
Reported by Scott Harrison.