By default, in Debian and its derivatives, 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,
install a shell-script that sources /etc/default/shorewall and honours
`SAFESTOP` when stopping Shorewall and patch the service files to call
it.
Signed-off-by: Jeremy Sowden <jeremy@azazel.net>