MINOR: mux-fcgi: make sure any stream always has an endpoint

MINOR: mux-fcgi: make sure any stream always has an endpoint

The principle that each mux stream should have an endpoint is not
guaranteed for closed streams that map to the dummy static streams.
Let's have a dummy endpoint for use with such streams. It only has
the DETACHED flag and a NULL conn_stream, and is referenced by all
the closed streams so that we can afford not to test strm->endp when
trying to access the flags or the CS.