AuthManager fixups around the login→RESTART→create flow

AuthManager fixups around the login→RESTART→create flow

* ApiQueryAuthManagerInfo will differentiate between preserved linking
  data and a preserved createRequest.
* ApiQueryAuthManagerInfo will indicate the preserved username, if any,
  because the client will have to pass that back to action=createaccount.
* ApiClientLogin won't tell about the confusing
  CreateFromLoginAuthenticationRequest returned on RESTART responses.
* Explain how 'preservestate' works in ApiAMCreateAccount's auto-doc.
* ConfirmLinkSecondaryAuthenticationProvider will filter out requests
  that can no longer be used (i.e. if it was for linking the account
  that got used for creation).
* All the complicated code in AuthManager::beginAccountCreation() was
  trying to deal with allowing the client to pass only the
  CreateFromLoginAuthenticationRequest. That was dumb, removed it.
* Added methods to CreateFromLoginAuthenticationRequest to indicate its
  status with respect to different kinds of preserved state.
* Increase accuracy of the AuthenticationResponse::$createRequest doc.

Change-Id: I726d79de18e739d6e60c1eea51453433c21ba207