Allow modules to alter the redirect url

It can be convenient to allow other modules to alter aspects of the url that a securepages redirect redirects to.

A typical example of where this is useful is if you need to notify the target page (either by adding a query parameter, or adding some extra data) that the user arrived from a change of protocol by securpages. If this alter hook were in place, then doing so would require just a line or 2 of code. Doing this kind of thing without the alter hook is unreliable and hacky. It could also be used to redirect to specific pages customized for HTTP or HTTPS.

Attaching a possible patch.