Add the ability to register filters. This commit introduces a warning

into the build.  This warning will be removed automatically, as soon as
we decide on a prototype for the function causing the warning.  That
decision is tied to which filtering mechanism we decide on.
Submitted by:	Ryan Bloom and Greg Stein


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@85926 13f79535-47bb-0310-9956-ffa450edef68