postrm: add script to clean systemd on uninstall

This script stops and disables the dnc.service unit file and removes it.
It's a fix for an issue where, on upgrade or uninstall+install, the
service file became 'masked' and linked to /dev/null.

Change-Id: Ifcc6bc42212e0241c775cdd24df5f937780e0a9b

extras/packaging/gnu-linux/debian/postrm

+#!/bin/sh
+
+(test -x /usr/bin/systemctl && systemctl stop dnc.service && systemctl disable dnc.service && echo "stopped dnc.service") || true
+
+(test -e /etc/systemd/system/dnc.service && rm -f /etc/systemd/system/dnc.service) || true
+
+exit 0