MINOR: mux-fcgi/trace: Register a new trace source with its events

MINOR: mux-fcgi/trace: Register a new trace source with its events

As for the mux h1 and h2, traces are now supported in the mux fcgi. All parts of
the multiplexer is covered by these traces. Events are splitted by categories
(fconn, fstrm, stream, rx, tx and rsp) for a total of ~40 different events with
5 verboisty levels.

In traces, the first argument is always a connection. So it is easy to get the
fconn (conn->ctx). The second argument is always a fstrm. The third one is an
HTX message. 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.