Clean up md5 digest support in default_handler() a bit:

1) don't compute md5 if we have a content filter; it is almost always wrong
2) add note about adding md5 filter in the future; this can be done
3) delete some APACHE_XLATE/CHARSET_EBCDIC baggage associated with md5
   digest support; part of this deleted baggage is a segfault on
   subrequests referencing r->rrx (which is not allocated for subrequests)


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@86385 13f79535-47bb-0310-9956-ffa450edef68