Pass srcdump request parameter when redirecting etc.

For most of CAF's purposes, the srcdump request parameter is not
really for srcdump, since it is not related to authentication.

Rather, it exists simply because we do not own the application path
namespace.  So when generating (or requesting) redirects etc. we
should treat it as a form parameter relating to the application.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>