maintainer scripts: use invoke-rc.d instead of directly invoking init script

Addressing lintian errors:

| E: ngcp-mediaproxy-ng-kernel-dkms: maintainer-script-calls-init-script-directly postinst:22
| E: ngcp-mediaproxy-ng-kernel-dkms: maintainer-script-calls-init-script-directly prerm:12

From: Michael Prokop <mprokop@sipwise.com>

debian/ngcp-mediaproxy-ng-kernel-dkms.postinst

@@ -19,7 +19,7 @@ if [ "$1" = 'configure' ] ; then
 
         # try to start the daemon
         if [ -x /etc/init.d/ngcp-mediaproxy-ng-daemon ] ; then
-                /etc/init.d/ngcp-mediaproxy-ng-daemon start || true
+		invoke-rc.d ngcp-mediaproxy-ng-daemon start || true
         fi
 fi
 

debian/ngcp-mediaproxy-ng-kernel-dkms.prerm

@@ -9,7 +9,9 @@ version=`dpkg-query -W -f='${Version}' "$package" \
         |rev|cut -d- -f2-|rev|cut -d':' -f2|tr -d "\n"`
 
 # make sure it's not running
-/etc/init.d/ngcp-mediaproxy-ng-daemon stop || true
+if [ -x /etc/init.d/ngcp-mediaproxy-ng-daemon ] ; then
+	invoke-rc.d ngcp-mediaproxy-ng-daemon stop || true
+fi
 
 dkms remove -m "$name" -v "$version" --all || true