phabricator: migrate [phabricator.auth] to [auth] This resurrects the warning mechanism removed in 20a4543e9a2b. I'm not worried about the copy/paste of the code for a brief transitional period.

dispatch: unify handling of None returned by a command function A command function may return None in place of 0 just for convenience, but dispatch() doesn't need to inherit that property. This patch makes it be friendly to callers.