BUG/MAJOR: mux-h2: split absolute URIs like the HTTP analysers do on output

BUG/MAJOR: mux-h2: split absolute URIs like the HTTP analysers do on output

When deriving :authority and :path from an absolute-form URI, the
authority ended at the first '/' only instead of the first '/', '?' or
'#' (RFC3986#3.2), and the userinfo was stripped up to the first '@'
instead of the last one. The analysers and sample fetches rely on
http_parse_authority() which does it correctly, so a request could be
evaluated for one host/path and delivered to the H2 server with
another one, bypassing host and path based rules.

Now the authority is extracted like http_parse_authority() does, and a
'/' is prepended to :path when needed.

This exists since commit b8ce8905c ("MEDIUM: mux-h2: do not map Host to
:authority on output") in 2.1, the userinfo part since 29fa2667a
("BUG/MINOR: mux-h2: strip the userinfo when deriving :authority for a
server") in 3.5-dev5. Must be backported to all stable versions (the
userinfo part only where 29fa2667a was backported).