Followup on previous commits. Use redirectUntrustedURL() as a shortcut, and let everything else make use of redirectTrustedURL(). Move the responsibility to check the input out of the library, to the places where URLs are grabbed from input parameters.

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@3332 44740490-163a-0410-bde0-09ae8108e29a