Do not fail on missing files in /etc/, create them instead.

passwd, shadow, group, gshadow etc. can be managed via nss -
e.g. system default accounts can be specified using nss_altfiles,
rather than in /etc/. Thus despite having default accounts, these
files can be missing on disk and thus should be opened with O_CREATE
whenever they are attempted to be opened in O_RDWR modes.

src/grpunconv.c

@@ -166,7 +166,7 @@ int main (int argc, char **argv)
 		fail_exit (5);
 	}
 	gr_locked = true;
-	if (gr_open (O_RDWR) == 0) {
+	if (gr_open (O_CREAT | O_RDWR) == 0) {
 		fprintf (stderr,
 		         _("%s: cannot open %s\n"), Prog, gr_dbname ());
 		fail_exit (1);