Add missing variables:

Introduced in r1132494
   CONTEXT_PREFIX
   CONTEXT_DOCUMENT_ROOT
and since the beginning of util_expr_eval.c  for
   HTTP_COOKIE

Description still missing.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1595348 13f79535-47bb-0310-9956-ffa450edef68