2011/04/20 - List of keep-alive / close options with associated behaviours.

PK="http-pretend-keepalive", HC="httpclose", SC="http-server-close",
FC = "forceclose".

0 = option not set
1 = option is set
* = option doesn't matter

Options can be split between frontend and backend, so some of them might have
a meaning only when combined by associating a frontend to a backend. Some forms
are not the normal ones and provide a behaviour compatible with another normal
form. Those are considered alternate forms and are marked "(alt)".

FC SC HC PK    Behaviour
 0  0  0  X    tunnel mode
 0  0  1  0    passive close, only set headers then tunnel
 0  0  1  1    forced close with keep-alive announce (alt)
 0  1  0  0    server close
 0  1  0  1    server close with keep-alive announce
 0  1  1  0    forced close (alt)
 0  1  1  1    forced close with keep-alive announce (alt)
 1  *  *  0    forced close
 1  *  *  1    forced close with keep-alive announce

At this point this results in 4 distinct effective modes for a request being
processed :
  - tunnel mode   : Connection header is left untouched and body is ignored
  - passive close : Connection header is changed and body is ignored
  - server close  : Connection header set, body scanned, client-side keep-alive
                    is made possible regardless of server-side capabilities
  - forced close  : Connection header set, body scanned, connection closed.

The "close" modes may be combined with a fake keep-alive announce to the server
in order to workaround buggy servers that disable chunked encoding and content
length announces when the client does not ask for keep-alive.

Note: "http-pretend-keepalive" alone has no effect. However, if it is set in a
      backend while a frontend is in "http-close" mode, then the combination of
      both will result in a forced close with keep-alive announces for requests
      passing through both.

It is also worth noting that "option httpclose" alone has become useless since
1.4, because "option forceclose" does the right thing, while the former only
pretends to do the right thing. Both options might get merged in the future.