Refactor scoping code in the SP class

Make sure the `saml:IDPList` and `IDPList` parameters are not overloaded. From now on, saml:IDPList is being used when SSP is used as a proxy while the IDPList parameter is used when SSP is used as in SP-mode and you want to apply scoping to your AuthnRequest.

See https://github.com/simplesamlphp/simplesamlphp/pull/1563 for the full explanation of the changes.