This patch removes the processing of `mxb' parameters in Accept

headers in mod_negotiation.  A second patch updates the manual to
reflect this (mxb is not documented directly in the manual but support
for it is implied in one place).

Reasons for removing this feature:

1) As currently implemented, the 'mxb' feature makes possible certain
denial-of-service attacks on negotiated content.  These attacks are
posssible for user communities which access an Apache server from
behind a HTTP/1.1 proxy which implements `Vary' related optimisations.
Plugging this denial of service hole without removing `mxb' is fairly
expensive in terms of degrading caching efficiency.

2) `mxb' is not in HTTP/1.0 or HTTP/1.1 or any other standard

3) Nobody seems to make use of 'mxb'.  (Balachander Krishnamurthy
kindly offered to grep some of his web traffic traces -- he did not
find a single Accept with mxb in a whole day of recent traffic, nor in
older traces)

4) Removing a feature makes a nice change from adding features.

Submitted by:	Koen Holtman <Koen.Holtman@cern.ch>


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@83288 13f79535-47bb-0310-9956-ffa450edef68