MEDIUM: connections: Use both mux_proto and alpn to pick a mux

MEDIUM: connections: Use both mux_proto and alpn to pick a mux

In conn_get_best_mux() and conn_get_best_mux_entry(), the mux name was
provided sometimes based on the "proto" directive, sometimes based on
the ALPN, but in any case, it was compared again the mux_proto_list
mux_proto field. This is not correct, as ALPN can be different from the
internal mux_proto. So enhance those functions so that they wll accept
an ALPN as well. If a mux_proto is provided, that will be used, if not,
and if an ALPN is provided, then that will be used, and compared against
the ALPN provided by the mux, if any.