Refactoring/deprecating of Yast::Service

Goals of this PR:
* Make `Yast::Service` compatible with systemd to use `Yast::SystemdService` backend
* Deprecate old unsupported API

This includes changes to code for systemd backend due to compatibility/improvements .

This PR is missing new tests for `Yast::Service`. I will add them if the new code is reviewed, and I will remove the old yast testsuite for it as well.