idp/saml: set signature verify hint when verifying logout responses
idp/saml: set signature verify hint when verifying logout responses

It uses the authn_request_signature_check_hint. A custom field
like logout_request_signature_check_hint should be used in the future.

pu239ppy/authentic2,pu239ppy/authentic2,BryceLohr/authentic,adieu/authentic2,adieu/authentic2,adieu/authentic2,pu239ppy/authentic2,pu239ppy/authentic2,BryceLohr/authentic,adieu/authentic2,BryceLohr/authentic,BryceLohr/authentic