usermod: Return error if -L and -U options are used simultaneously

Author: tcl3

Userland/Utilities/usermod.cpp

@@ -1,5 +1,6 @@
 /*
  * Copyright (c) 2021, Brandon Pruitt <brapru@pm.me>
+ * Copyright (c) 2023, Tim Ledbetter <timledbetter@gmail.com>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -50,6 +51,12 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
 
     args_parser.parse(arguments);
 
+    if (lock && unlock) {
+        warnln("The -L and -U options are mutually exclusive");
+        args_parser.print_usage(stderr, arguments.strings[0]);
+        return 1;
+    }
+
     auto account_or_error = Core::Account::from_name(username);
 
     if (account_or_error.is_error()) {