lib/, libsubid/, po/, src/: get_gid(): Move function to "atoi/getnum.h"

Implement it as an inline function, and add restrict and ATTR_STRING()
and ATTR_ACCESS() as appropriate.

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

lib/prototypes.h

@@ -142,10 +142,6 @@ extern int find_new_sub_gids (gid_t *range_start, unsigned long *range_count);
 extern int find_new_sub_uids (uid_t *range_start, unsigned long *range_count);
 #endif				/* ENABLE_SUBIDS */
 
-
-/* get_gid.c */
-extern int get_gid (const char *gidstr, gid_t *gid);
-
 /* getgr_nam_gid.c */
 extern /*@only@*//*@null@*/struct group *getgr_nam_gid (/*@null@*/const char *grname);