Add the AddFilter directive. This directive takes a list of filter names

Add the AddFilter directive.  This directive takes a list of filter names
that have been previously registered with the server.  Currently the
directive is only valid inside the config file, but once the Options
directive is tweaked a bit, I would feel more comfortable exposing this
directive to htaccess files.

As a part of making adding this filter, I removed the ctx pointer from the
ap_add_filter prototype.  The problem is that the core is the thing that
is actually inserting the filter into the filter stack, but the core doesn't
know how to allocate memory for each filter.  The solution is to have the
filters themselves be responsible for allocating the ctx memory whenever
it is required.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@86220 13f79535-47bb-0310-9956-ffa450edef68