LibCore: Properly shutdown a socket on Windows

It fixes a bug in which ImageDecoder and RequestServer
do not exit because their connections don't close.

This makes the shutdown behavior match the Linux version,
which receives FD_READ | FD_HANGUP on socket close, and
TransportSocket::read_as_much_as_possible_without_blocking calls
schedule_shutdown when read from a socket returns 0 bytes.

On Windows, we have to explicitly call WIN32 shutdown to receive
notification FD_CLOSE.

Libraries/LibCore/SocketWindows.cpp

@@ -122,6 +122,8 @@ void PosixSocketHelper::close()
     if (m_notifier)
         m_notifier->set_enabled(false);
 
+    // shutdown is required for another end to receive FD_CLOSE
+    shutdown(m_fd, SD_BOTH);
     MUST(System::close(m_fd));
     m_fd = -1;
 }