MEDIUM: htx: Allow the option http-use-htx to be used on TCP proxies too

MEDIUM: htx: Allow the option http-use-htx to be used on TCP proxies too

This will be mandatory to allow upgrades from TCP to HTTP in HTX. Of course, raw
buffers will still be used by default on TCP proxies, this option sets or
not. But if you want to handle mux upgrades from a TCP proxy, you must enable
the HTX on it and on all its backends.

There is only a small change in the lua code. Because TCP proxies can be HTX
aware, to exclude TCP services only for HTTP proxies, we must also check the
mode (TCP/HTTP) now.