MEDIUM: mux_pt: make cs_shutr() / cs_shutw() properly close the connection

MEDIUM: mux_pt: make cs_shutr() / cs_shutw() properly close the connection

Now these functions are able to automatically close both the transport
and the socket layer, causing the whole connection to be torn down if
needed.

The two shutdown modes are implemented for both directions, and when
a direction is closed, if it sees the other one is closed as well, it
completes by closing the connection. This is similar to what is performed
in the stream interface.

It's not deployed yet but the purpose is to get rid of conn_full_close()
where only conn_stream should be known.