FV2 breaking only works on 10.8; add checks and revert to launchd disable

On 10.9 and later, there's no mechanism to add local users via fdesetup without
providing an existing user's password or recovery key. On non-10.8 systems,
fall through to moving launchd out of the way.
Merge pull request #6 from ccaviness/master

FV2 breaking only works on 10.8; add check and revert to launchd disable