Make ap_add_filter use a LIFO stack instead of a FIFO queue to add filters

Make ap_add_filter use a LIFO stack instead of a FIFO queue to add filters
to the filter stack.  This makes the chunking filter work.  Without this,
the core_filter is installed in the insert_filters hook, when we get
to the ap_send_http_headers function, we insert the chunking filter, but
it has been installed after the core_filter.  This means the chunk_filter
is never called.

This reverses this.  The core_filter is installed in the insert_filters
hook, and we put the chunk_filter in during ap_send_http_headers.  This
time, the chunking filter is installed before the core_filter, and it
gets called correctly.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@86089 13f79535-47bb-0310-9956-ffa450edef68