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.

Maybe I'm just picky, but I'd rather see this whole block go away, and the below `if ( systemdRunning() )`  block go into its own sub, which is called from the existing `if ( $command` blocks.  Otherwise looks OK.