MINOR: systemd: Make use of master socket in systemd unit

Unless the EXTRAOPTS variable is overriden in /etc/default/haproxy
the unit file will use the master socket by default.

This patch may be backported to 1.9 and depends on
MINOR: systemd: Use the variables from /etc/default/haproxy.
diff --git a/contrib/systemd/haproxy.service.in b/contrib/systemd/haproxy.service.in
index 1387012..44c369c 100644
--- a/contrib/systemd/haproxy.service.in
+++ b/contrib/systemd/haproxy.service.in
@@ -4,7 +4,7 @@
 
 [Service]
 EnvironmentFile=-/etc/default/haproxy
-Environment="CONFIG=/etc/haproxy/haproxy.cfg" "PIDFILE=/run/haproxy.pid"
+Environment="CONFIG=/etc/haproxy/haproxy.cfg" "PIDFILE=/run/haproxy.pid" "EXTRAOPTS=-S /run/haproxy-master.sock"
 ExecStartPre=@SBINDIR@/haproxy -f $CONFIG -c -q $EXTRAOPTS
 ExecStart=@SBINDIR@/haproxy -Ws -f $CONFIG -p $PIDFILE $EXTRAOPTS
 ExecReload=@SBINDIR@/haproxy -f $CONFIG -c -q $EXTRAOPTS