Fix check_is_enabled for recent systemd

Recent systemd outputs extra BLACK_CIRCLE mark on list-dependencies like below.

```
% systemctl list-dependencies --plain sysinit.target                                                                                                                                                                                                                                                  [~/git]
sysinit.target
* dev-hugepages.mount
* dev-mqueue.mount
* kmod-static-nodes.service
* ldconfig.service
...
```

Not that the mark may vary depending on the locale.
http://cgit.freedesktop.org/systemd/systemd/tree/src/shared/util.c?id=v215#n5468
Merge pull request #212 from eagletmt/fix-is_enabled-on-systemd

Fix check_is_enabled for recent systemd