LinuxService: systemd related improvements

 - check if systemd is really in running, not just installed
 - instead of listing all the unit files, verify only the current unit
 - simplify/shorten code

Basically do things by the book in a way similar with what systemd does.
Will different distros put this in different places?  Do we need to check both /run/systemd and /var/run/systemd, for instance?