Add a handler to mod_includes.c. This handler is designed to

implement the XbitHack directive.  This can't be done with a
fixup, because we need to check the content-type, which is
only available in the handler phase.
PR:     7751


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@89872 13f79535-47bb-0310-9956-ffa450edef68