Add systemd service support

* needed by fate#314946

```ruby
require 'yast'
Yast.import 'SystemdService'
service = Yast::SystemdService.find('sshd')
 # or using the full unit id
service = Yast::SystemdService.find('sshd.service')
service.unit_name   # 'sshd'
service.unit_type   # 'service'
service.id          # 'sshd.service'
service.description # 'OpenSSH Daemon'
service.running?    # true if it's active and running
# and so on
```