src/: Recommend --badname only if it is useful

(Review with -w (--ignore-all-space).)

Closes: <https://github.com/shadow-maint/shadow/issues/1067>
Reported-by: Anselm Schüler <mail@anselmschueler.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>

src/useradd.c

@@ -1523,10 +1523,16 @@ static void process_flags (int argc, char **argv)
 		}
 
 		user_name = argv[optind];
-		if (!is_valid_user_name (user_name)) {
-			fprintf (stderr,
-			         _("%s: invalid user name '%s': use --badname to ignore\n"),
-			         Prog, user_name);
+		if (!is_valid_user_name(user_name)) {
+			if (errno == EINVAL) {
+				fprintf(stderr,
+				        _("%s: invalid user name '%s': use --badname to ignore\n"),
+				        Prog, user_name);
+			} else {
+				fprintf(stderr,
+				        _("%s: invalid user name '%s'\n"),
+				        Prog, user_name);
+			}
 #ifdef WITH_AUDIT
 			audit_logger (AUDIT_ADD_USER, Prog,
 			              "adding user",