When accidentally misspelling a name, privileges are sometimes granted to non-existent players, leaving them with the extra privileges, but without the default privileges, if they ever join the server. This corrects that by disallowing /grant from working with invalid players. For completeness, it does the same for /revoke.

According to #253, using `/clearpassword` without an argument causes the server to crash from an assertion failure. I've resubmitted matttpt's patch as a pull request to aid in merging.

Commands moved:
 /me
 /status
 /time
 /shutdown
 /ban
 /clearobjects