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.
No, unless they fork systemd and modify its paths.
For example Debian has migrated to `/run` according to their [release notes][1].

[1]: https://www.debian.org/releases/stable/i386/release-notes/ch-whats-new.en.html#tmpfs-filesystems