TCP: attempt to flush the write buffers before closing

When a socket is closed, it should make sure that any pending write
data is sent before the FIN is sent.  It already would wait for all
sent data to be acked, however it would discard any pending write
data that had not been sent at least once.

This change adds a check for pending write data in addition to unacked
data.  However, to be able to actually send any new data, the send
callback must be left.  The callback should be freed later when the socket
is actually destroyed.