Begin mod_core.h for CORE-private information. Ideally, AP_CORE_DECLARE

function decl's and CORE_PRIVATE header info should all move into this
header.

Start with moving the filter function declarations.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@87808 13f79535-47bb-0310-9956-ffa450edef68