Don't check for zmdc.pl when stopping via systemd

If this check ever actually returns true, it makes ExecStop a no-op
and systemd will forcefully kill all processes in the service
see man systemd.service

This also prevents a deadlock when running systemctl stop,
zmpkg.pl detects bash as it's parent and attempts to call
systemctl stop itself, which blocks forever waiting for the
task it's in to complete
Merge pull request #647 from ariscop/master

Don't check for zmdc.pl when stopping via systemd