Add a filter_init function to the filters so that a filter can execute

Add a filter_init function to the filters so that a filter can execute
arbitrary code before the handlers are invoked.

This resolves an issue with incorrect 304s on If-Modified-Since mod_include
requests since ap_meets_conditions() is not aware that this is a dynamic
request and it is not possible to satisfy 304 for these requests (unless
xbithack full is on, of course).  When mod_include runs as a filter, it is
too late to set any flag since the handler is responsible for calling
ap_meets_conditions(), which it should do before generating any data.

If a module doesn't need to run such arbitrary code, it can just pass NULL
as the argument and all is well.

PR:	9673
Reviewed by:	Ryan Bloom and others

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@95906 13f79535-47bb-0310-9956-ffa450edef68