BUG/MEDIUM: h3: split absolute URIs like the HTTP analysers do on output

BUG/MEDIUM: h3: split absolute URIs like the HTTP analysers do on output

h3_req_headers_send() derived :authority and :path from absolute-form
URIs with the same flaw as the H2 mux, fixed by the previous commit:
the authority ended at the first '/' instead of the first '/', '?' or
'#' (RFC3986#3.2), the userinfo was not stripped, and an empty path
was sent as an empty :path. A request evaluated by haproxy for one
host/path could be delivered to an H3 server with another one,
bypassing host and path based rules.

Now the authority is extracted like http_parse_authority() does, the
userinfo is stripped, and :path always starts with a '/' (or is '*'
for OPTIONS). As H3 backends are still experimental, the impact is
limited.

This was introduced by commit 235e818fa ("MINOR: h3: complete HTTP/3
request scheme encoding") in 3.3. Must be backported as far as 3.3.