Support `SAFESTOP` under systemd in Debian
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.