proxy HTTP: Rework the handling of request bodies to handle

chunked input and input filters which modify content length, and
avoid spooling arbitrary-sized request bodies in memory.

This is a merge of the proxy-reqbody branch into the trunk.

Justin had the protocol issues resolved in 2.1-dev, but not the 
memory issue.  The new implementation makes an effort to
stream the request body whenever possible.

Thanks to Allan Edwards for a critical bug fix, Greg Ames
for some style corrections, Justin for a needed commentary
addition, Jan Kratochvil for testing the analogous 2.0.x patch 
with the SonyEricsson P900 phone, and reviews by all these 
folks + JimJag.

PR: 15859


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@151168 13f79535-47bb-0310-9956-ffa450edef68