Fix systemd check in postinst. Closes: #829418

debian/pptpd.postinst

@@ -5,8 +5,8 @@ set -e
 case "$1" in
 configure)
     # load the module configured in /etc/modules-load.d/pptpd.conf
-    if [ -x "$(which systemctl || true)" ] ; then
-        systemctl restart systemd-modules-load
+    if [ -d /run/systemd/system ] ; then
+        deb-systemd-invoke restart systemd-modules-load.service
     else
         modprobe nf_nat_pptp 2>/dev/null || true
     fi