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>

src/newusers.c

@@ -31,6 +31,7 @@
 #include <string.h>
 
 #include "alloc.h"
+#include "atoi/getnum.h"
 #include "atoi/str2i.h"
 #ifdef ACCT_TOOLS_SETUID
 #ifdef USE_PAM