Fix services.udisks.enable.

Latest update to udisks in 344f2e65 broke it for me. Fix it by doing the
following:

- Add udisks.service to /etc/systemd/system (via systemd.packages)
- Fix path to udisks-daemon in udisks.service (libexec/ instead of lib/)
Merge pull request #2032 from the-kenny/fix-udisks1

Fix services.udisks.enable = true