BUG/MEDIUM: cli: _getsocks must send the peers sockets

BUG/MEDIUM: cli: _getsocks must send the peers sockets

This bug prevents to reload HAProxy when you have both the seamless
reload (-x / expose-fd listeners) and the peers.

Indeed the _getsocks command does not send the FDs of the peers
listeners, so if no reuseport is possible during the bind, the new
process will fail to bind and exits.

With this feature, it is not possible to fallback on the SIGTTOU method
if we didn't receive all the sockets, because you can't close() the
sockets of the new process without closing those of the previous
process, they are the same.

Should fix bug #443.

Must be backported as far as 1.8.