Fix OpenID authentication problems with myopenid.com etc.

OpenID authentication responses from an OpenID Provider (OP) can not only come via HTTP Redirect/GET (e.g. Google), but also via HTML FORM Redirection/POST (e.g. myopenid.com), which the current implementation does not support.

This update changes the decision criteria of whether a request is coming from an OP or not to testing for the presence of the parameter "openid.mode", which is a required filed for [all OpenID HTTP messages](https://openid.net/specs/openid-authentication-2_0.html#http_encoding). (Not using "openid.ns" because it is not present in OpenID 1.1.)

See also https://openid.net/specs/openid-authentication-2_0.html