Merge r1235019, r1236122, r1240470:

    Make the core input/output filter contexts private and provide a hook
    to insert a custom bucket, for use bye mpm_winnt and mod_ftp.
    
    This allows to add members to the context structs without breaking binary
    compatibility.

This should be ABI back-compatible, so only a minor MMN bump.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1240478 13f79535-47bb-0310-9956-ffa450edef68