Various fixes for CoreOS distro

- Clean up service unit
- Add /etc/machine-id to deprovision blacklist
- Add custom RestartInterface()
- Stop networkd before running DHCP client
- Remove install/uninstall register/deregister functions
- Make sure that newly-created user accounts aren't locked
Refactor DHCP start/stop into its own methods
Merge pull request #45 from crawford/coreos

Refactor DHCP start/stop into its own methods
Merge branch 'crawford-dhcp' into 2.0