MINOR: config: make sure to associate the proper mux to bind and servers

MINOR: config: make sure to associate the proper mux to bind and servers

Currently a mux may be forced on a bind or server line by specifying the
"proto" keyword. The problem is that the mux may depend on the proxy's
mode, which is not known when parsing this keyword, so a wrong mux could
be picked.

Let's simply update the mux entry while checking its validity. We do have
the name and the side, we only need to see if a better mux fits based on
the proxy's mode. It also requires to remove the side check while parsing
the "proto" keyword since a wrong mux could be picked.

This way it becomes possible to declare multiple muxes with the same
protocol names and different sides or modes.