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.
diff --git a/contrib/systemd/haproxy.service.in b/contrib/systemd/haproxy.service.in
index dca81a2..05bb716 100644
--- a/contrib/systemd/haproxy.service.in
+++ b/contrib/systemd/haproxy.service.in
@@ -3,6 +3,8 @@
 After=network.target
 
 [Service]
+# You can point the environment variable HAPROXY_STATS_SOCKET to a stats
+# socket if you want seamless reloads.
 Environment="CONFIG=/etc/haproxy/haproxy.cfg" "PIDFILE=/run/haproxy.pid"
 ExecStartPre=@SBINDIR@/haproxy -f $CONFIG -c -q
 ExecStart=@SBINDIR@/haproxy-systemd-wrapper -f $CONFIG -p $PIDFILE