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>

libsubid/api.c

@@ -12,11 +12,13 @@
 #include <string.h>
 #include <pwd.h>
 #include <stdbool.h>
+
 #include "subordinateio.h"
 #include "idmapping.h"
 #include "subid.h"
 #include "shadowlog.h"
 
+
 bool subid_init(const char *progname, FILE * logfd)
 {
 	FILE *shadow_logfd;