[MEDIUM] move all HTTP Request-related session material to struct hreq

[MEDIUM] move all HTTP Request-related session material to struct hreq

The req_cap, hdr_state, hdr_idx, auth_hdr and req_line have been moved
to a dedicated hreq structure in the session. It makes is easier to
add HTTP-specific fields such as SOR (start of request) and EOF (end
of headers).

It also made it possible to fix two bugs introduced by last commit :
 - end of headers not correctly detected
 - hdr_idx not freed upon one specific error during session creation

When the backend side will be reworked, it should rely on a similar
structure.