Improves the OpenID support in plat/al:

Improves the OpenID support in plat/al:
 * fixes a few security issues (including an XSRF vulnerability);
 * simplifies the authorization logic (now using the one advised by Auth::OpenID);
 * stops using the session to store OpenID requests (everything is passed in the URL);
 * simplifies the design of the trust page, and allows users to disable SReg.

Signed-off-by: Vincent Zanotti <vincent.zanotti@m4x.org>