CtdlCheckExpress() now accepts a session context.

This is an experiment in reducing the use of the "CC" macro
that appears everywhere in the server.  Willi used to do a lot
of "struct CitContext *CCC = CC" but that still called the macro
once per function call and it added complexity to the code.  This
method is the right way, actually passing the session context up
the stack like we do in WebCit-NG.