systemd: check multi-user.target instead of default.target

The behavior of `default.target` is a bit special and doesn't report as
active consistently even though it is an alias for `multi-user.target`
which is indeed active. Not really worth any more of our time figuring
out why this is the case so just switch the check.

Required for systemd 217.
Merge pull request #41 from marineam/systemd

systemd: check multi-user.target instead of default.target