LocalSocket: Teach recvfrom() how to block if needed, and simplify it

If we can't already read when we enter recvfrom() on a LocalSocket,
we'll now block the current thread until we can.

Also added a buffer_for(FileDescription&) helper so that the client
and server can share some of the code. :^)

Kernel/Net/LocalSocket.h

@@ -36,6 +36,7 @@ private:
     virtual bool is_local() const override { return true; }
     bool has_attached_peer(const FileDescription&) const;
     static Lockable<InlineLinkedList<LocalSocket>>& all_sockets();
+    DoubleBuffer& buffer_for(FileDescription&);
 
     // An open socket file on the filesystem.
     RefPtr<FileDescription> m_file;