Login fails with both STRICT_USERNAMES and FOLLOW_RENAMES enabled

[Michael Hall]

When both OPENID_STRICT_USERNAME and OPENID_FOLLOW_RENAMES are enabled, and the user is logging in for a second time, it will tell the user that someone else is using their username.

This is because OPENID_FOLLOW_RENAMES will call _get_available_username() on the returned openid nickname. Inside there, OPENID_STRICT_USERNAME will throw an error if it finds a user already using that username (without checking that the user it finds is the currently logging in user)