full systemd support

This method combines ideas from the previous two pull requests.  zmpkg will automatically detect that systemd is running, and it will automatically redirect any start/stop/restart commands through systemctl.

I totally agree with this, but I think I'd rather do that in a separate pull request.  If you look at the entire zmpkg, I'd argue that every single one of the `if ( $command` blocks should be in its own sub.  So if I can get you to agree to compromise, I'd ask you to merge the systemd pull request first, and then I'll open new pull request after I re-write the entire zmpkg.