Support `SAFESTOP` under systemd in Debian

Jeremy Sowdenrequested to merge
a3a3el/shorewall-code:support-SAFESTOP-under-systemd-in-debian/v1 into master

By default, stopping the Shorewall service executes /sbin/shorewall clear.

The SAFESTOP setting in /etc/default/shorewall is intended to stop the service by calling /sbin/shorewall stop.

However, the systemd service files do not support this. Instead, we install a shell-script that sources /etc/default/shorewall and honours SAFESTOP when stopping Shorewall and patch the service files to call it.

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