MEDIUM: mux-pt: Expose passthrough in the list of supported mux protocols

MEDIUM: mux-pt: Expose passthrough in the list of supported mux protocols

Add "none" in the list of supported mux protocols. It relies on the
passthrough multiplexer and use almost the same mux_ops structure. Only the
flags differ because this "new" mux does not support the upgrades. "none"
was chosen to explicitly stated there is not processing at the mux level.

Thus it is now possible to set "proto none" or "check-proto none" on
bind/server lines, depending on the context. However, when set, no upgrade
to HTTP is performed. It may be a way to disable HTTP upgrades per bind
line.