mkfifo: Don't rely on global errno

Core::System::mkfifo() doesn't rely on POSIX's mkfifo() and sends the
syscall directly to our system. This means that the and errno doesn't
get updated which ultimately caused the program to display an incorrect
message 'mkfifo: Success (not an error)'.

Userland/Utilities/mkfifo.cpp

@@ -33,7 +33,7 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
     for (auto path : paths) {
         auto error_or_void = Core::System::mkfifo(path, mode);
         if (error_or_void.is_error()) {
-            perror("mkfifo");
+            warnln("mkfifo: Couldn't create fifo '{}': {}", path, error_or_void.error());
             exit_code = 1;
         }
     }