Fix issue with systemd template units which does not return properties;
this means running `systemctl show user@` returns an error instead of a property map
Update changes and version
Add missing makefiles
Make tests pass
Check for exit status, not stderr
Merge pull request #202 from vmoravec/fix-empty-properties

Fix issue with systemd template units

Our code relied on non-existent services returning a (bogus?) set of properties where `LoadState=not-found`. But `@` is a special case: if the instance identifier is missing, we get an early syntax error:
> \# systemctl show getty@.service
> Failed to get properties: Unit name getty@.service is not valid.