monkeysphere update-authorized-keys should use atomic moves
currently, it looks like @monkeysphere update-authorized-keys@ just redirects output into the designated authorized_keys file. This could be problematic if the update takes a long time while authentications are under way.
it would be better to create a new version and swap it in atomically, the way we do for @monkeysphere-authentication update-users@
(from redmine: created on 2010-10-16, closed on 2010-10-19)