Factor out the logic for handling systemd services to install-lib

[matteodelabre]

Originally posted by @matteodelabre in https://github.com/toltec-dev/toltec/pull/222#discussion_r559230038

[Eeems]

#721 implements disable-unit and unit-exists. We already have several other systemd service related method in install-lib. More could potentially be added though.