MINOR: mux: Unlink ALPN and multiplexers to rather speak of mux protocols

MINOR: mux: Unlink ALPN and multiplexers to rather speak of mux protocols

Multiplexers are not necessarily associated to an ALPN. ALPN is a TLS extension,
so it is not always defined or used. Instead, we now rather speak of
multiplexer's protocols. So in this patch, there are no significative changes,
some structures and functions are just renamed.