BUG/MEDIUM: mux-h1: Skip UNUSED htx block when formating the start line

BUG/MEDIUM: mux-h1: Skip UNUSED htx block when formating the start line

UNUSED blocks were not properly handled when the H1 multiplexer was
formatting the start line of a request or a response. UNUSED was ignored but
not removed from HTX message. So the mux can loop infinitly on such block.

It could be seen a a major issue but in fact it happens only if a very
specific case on the reponse processing (at least I think so): the server
must send an interim message (a 100-continue for intance) with the final
response. HAProxy must receive both in same time and the final reponse must
be intercepted (via a http-response return action for instance), In that
case, the interim message is fowarded and the server final reponse is
removed and replaced by a proxy error message.

Now UNUSED htx blocks are properly skipped and removed.

This patch must be backported as far as 3.0.