LibCore: Let LocalSocket send and receive messages with SCM_RIGHTS

These new methods combine send/receive with send_fd/receive_fd.

This is the 'correct' way to use SCM_RIGHTS, rather than trying to
emulate the Serenity behavior on other Unixes.

Userland/Libraries/LibCore/Socket.h

@@ -329,6 +329,10 @@ public:
 
     ErrorOr<int> receive_fd(int flags);
     ErrorOr<void> send_fd(int fd);
+
+    ErrorOr<Bytes> receive_message(Bytes buffer, int flags, Vector<int>& fds);
+    ErrorOr<ssize_t> send_message(ReadonlyBytes msg, int flags, Vector<int, 1> fds = {});
+
     ErrorOr<pid_t> peer_pid() const;
     ErrorOr<Bytes> read_without_waiting(Bytes buffer);