systemctl: fix operations on relative paths

systemctl: fix operations on relative paths

We should treat ./some.service and $PWD/some.service as equivalent. But we'd
try to send the relative paths over dbus, which can't work well:

$ sudo systemctl enable ./test2.service
Failed to look up unit file state: Invalid argument
$ sudo systemctl enable $PWD/test2.service
Created symlink /etc/systemd/system/multi-user.target.wants/test2.service → /home/zbyszek/src/systemd/test2.service.
Created symlink /etc/systemd/system/test2.service → /home/zbyszek/src/systemd/test2.service.

Now both are equivalent.