BUG/MEDIUM: server: Defer the mux init until after xprt has been initialized.

BUG/MEDIUM: server: Defer the mux init until after xprt has been initialized.

In connect_server(), if we're using a new connection, and we have to
initialize the mux right away, only do it so after si_connect() has been
called. si_connect() is responsible for initializing the xprt, and the
mux initialization may depend on the xprt being usable, as it may try to
receive data. Otherwise, the connection will be flagged as having an error,
and we will have to try to connect a second time.

This should be backported to 1.9.