BUG/MINOR: config: Be aware of the HTX during the check of mux protocols

BUG/MINOR: config: Be aware of the HTX during the check of mux protocols

Because the HTX is still experimental, we must add special cases during the
configuration check to be sure it is not enabled on a proxy with incompatible
options. Here, for HTX proxies, when a mux protocol is specified on a bind line
or a server line, we must force the HTX mode (PROTO_MODE_HTX).

Concretely, H2 is the only mux protocol that can be forced. And it doesn't yet
support the HTX. So forcing the H2 on an HTX proxy will always fail.