Allow filters to buffer data in a brigade using the ap_f* functions.

These have become simple macros that just wrap the apr_brigade functions,
allowing filter writers to ignore the flush function and the ctx pointer.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@88028 13f79535-47bb-0310-9956-ffa450edef68