saml: SAML 1.1: Prepare for IdP initiated authentication.

With IdP initiated authentication, we will not have the IdP entityID
available when receiving the response. This patch prepares the code for
that.

Unfortunately, the IdP entityID is required for the Artifact receiver,
so give an error when receiving an unsolicited response there.

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@2567 44740490-163a-0410-bde0-09ae8108e29a