MEDIUM: http: move Connection header processing earlier

MEDIUM: http: move Connection header processing earlier

Till now the Connection header was processed in the middle of the http-request
rules and some reqadd rules. It used to force some http-request actions to be
cut in two parts.

Now with keep-alive, not only that doesn't make any sense anymore, but it's
becoming a total mess, especially since we need to know the headers contents
before proceeding with most actions.

The real reason it was not moved earlier is that the "block" or "http-request"
rules can see a different version if some fields are changed there. But that
is already not reliable anymore since the values observed by the frontend
differ from those in the backend.

This patch is the equivalent of commit f118d9f ("REORG: http: move HTTP
Connection response header parsing earlier") but for the request side. It
has been tagged MEDIUM as it could theorically slightly affect some setups
relying on corner cases or invalid setups, though this does not make real
sense and is highly unlikely.