Allows accessing various keys, especially whether this is an
interactive login or not.
Also have the xml-rpc `login` delegate to `authenticate` instead of
having its own half-assed implementation.
And remove some dead code: as far as I can tell, Session.authenticate
is never called with a uid.
* Make Users._login and session.authenticate always raise AccessDenied
on authentication failure instead of only sometimes (cf
Session.authenticate calling security.check() which raises and not
catching the exception)
* Alter AccessDenied such that it's possible to add a custom access
message, for use with login rate limiting instead of smuggling the
information via the session
* Alter the RPC endpoints to catch and convert AccessDenied back to
a boolean sentinel
Xavier Morel