usermod: respect --prefix for --gid option

The --gid option accepts a group name or id. When a name is provided, it
is resolved to an id by looking up the name in the group database
(/etc/group).

The --prefix option overides the location of the passwd and group
databases. I suspect the --gid option was overlooked when wiring up the
--prefix option.

useradd --gid already respects --prefix; this change makes usermod
behave the same way.

Fixes: b6b2c756c9
Signed-off-by: Mike Gilbert <floppym@gentoo.org>

src/usermod.c

@@ -1072,7 +1072,7 @@ static void process_flags (int argc, char **argv)
 				fflg = true;
 				break;
 			case 'g':
-				grp = getgr_nam_gid (optarg);
+				grp = prefix_getgr_nam_gid (optarg);
 				if (NULL == grp) {
 					fprintf (stderr,
 					         _("%s: group '%s' does not exist\n"),