Detect goofy `sudo -n` output under OSX

Summary: See rP2fedb6f941d8. We might need a more general version of this since we do some `sudo` stuff elsewhere, but at least on my machine `sudo -n` exits with code 0 when the target user exists but needs a password.

Test Plan:
  - Tried to run daemons as root, with no automatic sudo to root. Got a bad result before (phd believed it had executed the daemons) and a good result afterward (phd recognized that sudo failed).
  - Tried to run daemons from root, as a non-root user. Got a good result in both cases.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: fabe, epriestley

Differential Revision: https://secure.phabricator.com/D11041