MINOR: mux_h2: define config to disable h2 websocket support

MINOR: mux_h2: define config to disable h2 websocket support

Define a new global config statement named
"h2-workaround-bogus-websocket-clients".

This statement will disable the automatic announce of h2 websocket
support as specified in the RFC8441. This can be use to overcome clients
which fail to implement the relatively fresh RFC8441. Clients will in
his case automatically downgrade to http/1.1 for the websocket tunnel
if the haproxy configuration allows it.

This feature is relatively simple and can be backported up to 2.4, which
saw the introduction of h2 websocket support.