[MAJOR] support for source binding via cttproxy

[MAJOR] support for source binding via cttproxy

Using the cttproxy kernel patch, it's possible to bind to any source
address. It is highly recommended to use the 03-natdel patch with the
other ones.

A new keyword appears as a complement to the "source" keyword : "usesrc".
The source address is mandatory and must be valid on the interface which
will see the packets. The "usesrc" option supports "client" (for full
client_ip:client_port spoofing), "client_ip" (for client_ip spoofing)
and any 'IP[:port]' combination to pretend to be another machine.

Right now, the source binding is missing from server health-checks if
set to another address. It must be implemented (think restricted firewalls).
The doc is still missing too.