[MAJOR] frontend: split accept() into frontend_accept() and session_accept()

[MAJOR] frontend: split accept() into frontend_accept() and session_accept()

A new function session_accept() is now called from the lower layer to
instanciate a new session. Once the session is instanciated, the upper
layer's frontent_accept() is called. This one can be service-dependant.

That way, we have a 3-phase accept() sequence :
  1) protocol-specific, session-less accept(), which is pointed to by
     the listener. It defaults to the generic stream_sock_accept().
  2) session_accept() which relies on a frontend but not necessarily
     for use in a proxy (eg: stats or any future service).
  3) frontend_accept() which performs the accept for the service
     offerred by the frontend. It defaults to frontend_accept() which
     is really what is used by a proxy.

The TCP/HTTP proxies have been moved to this mode so that we can now rely on
frontend_accept() for any type of session initialization relying on a frontend.

The next step will be to convert the stats to use the same system for the stats.