MINOIR: mux-pt/trace: Register a new trace source with its events

MINOIR: mux-pt/trace: Register a new trace source with its events

As for the other muxes, traces are now supported in the pt mux. All parts of
the multiplexer is covered by these traces. Events are splitted by
categories (connection, stream, rx and tx).

In traces, the first argument is always a connection. So it is easy to get
the mux context (conn->ctx). The second argument is always a conn-stream and
mau be NUUL. The third one is a buffer and it may also be NULL. Depending on
the context it is the request or the response. In all cases it is owned by a
channel. Finally, the fourth argument is an integer value. Its meaning
depends on the calling context.