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/groupmod.c

@@ -27,6 +27,7 @@
 #endif				/* ACCT_TOOLS_SETUID */
 
 #include "alloc.h"
+#include "atoi/getnum.h"
 #include "chkname.h"
 #include "defines.h"
 #include "groupio.h"
@@ -40,6 +41,8 @@
 #include "shadowlog.h"
 #include "string/stpecpy.h"
 #include "string/stpeprintf.h"
+
+
 /*
  * exit status values
  */