Fixes for RegisterThrottle plugin: alt registration methods (OpenID, FBConnect, Twitt...

Fixes for RegisterThrottle plugin: alt registration methods (OpenID, FBConnect, Twitter) weren't triggering the throttle check or recording of IPs.

Added StartRegistrationTry/EndRegistrationTry calls into those three, and moved the actual recording hook to EndUserRegister which is guaranteed to be called from User::register (so we don't need to worry about other auth methods forgetting to call the other UI-code hooks).