TT#53525 postinst: use service(8) to identify active kamailio-proxy instance

systemctl(1) (provided by systemd) isn't necessarily available in the
environment yet, e.g. as identified by piuparts tests.

Instead of relying on systemctl(1) let's use service(8) from
init-system-helpers (which is an essential package), and while at it
report the exit code of the app_lua reload.

Fixes:

| /var/lib/dpkg/info/lua-ngcp-kamailio.postinst: 7: /var/lib/dpkg/info/lua-ngcp-kamailio.postinst: systemctl: not found

Change-Id: I52cf30d6421f58c77ce1983d95c141d3a331e939

debian/lua-ngcp-kamailio.postinst

@@ -4,9 +4,10 @@ set -e
 
 case "$1" in
     configure)
-        if [ "$(systemctl is-active kamailio-proxy)" = "active" ] ; then
-            ngcp-sercmd proxy app_lua.reload 0 || true
-            echo "Kamailio proxy module 'app_lua' reloaded"
+        if service kamailio-proxy status >/dev/null 2>&1 ; then
+            echo "kamailio-proxy seems to be active, reloading app_lua"
+            ngcp-sercmd proxy app_lua.reload 0 || RC=$?
+            echo "Kamailio proxy module 'app_lua' reloaded (exit code: ${RC:-0})"
         fi
         ;;