MINOR: systemd wrapper: add support for passing the -x option.

MINOR: systemd wrapper: add support for passing the -x option.

Make the systemd wrapper chech if HAPROXY_STATS_SOCKET if set.
If set, it will use it as an argument to the "-x" option, which makes
haproxy asks for any listening socket, on the stats socket, in order
to achieve reloads with no new connection lost.