Security improvements and bug fixes

Improvements:

• driver no longer runs as root; it drops privileges and spawns a privileged subcommand only for those operations that absolutely need root.
• additional restrictions on hyperkit binary
  • if a hyperkit file exists in the same directory as the driver, than that is the only allowed version
  • the hyperkit executable must be owned by root, or group ownership must be by wheel or admin
• nfsd will be updated via restart instead of update, which works even if /etc/exports did not yet exist.