openid: Use Openid Simple Registration or OpenID Attribute Exchange to get the user...

The info is stored in the session database, not the user database.
There should be no reason to need it when a user is not logged in.

Also, hide the email field in the preferences page for openid users.

Note that the email and username are not yet actually used for anything.
The email will be useful for gravatar, while the username might be used
for a more pretty display of the openid.