user-accounts: Use absolute path of command usermod

On distro openSUSE Tumbleweed and SUSE Linux Enterprise the path
of command usermod is not in environment variable PATH, we need to
use absolute path to find command.

panels/user-accounts/user-utils.c

@@ -509,7 +509,7 @@ is_valid_username_async (const gchar *username,
          * future, so it would be nice to have some official way for this
          * instead of relying on the current "--login" implementation.
          */
-        argv[0] = "usermod";
+        argv[0] = "/usr/sbin/usermod";
         argv[1] = "--login";
         argv[2] = data->username;
         argv[3] = "--";