Follow up to r1877955: don't reuse the connection for mixed C-L / T-E requests

Follow up to r1877955: don't reuse the connection for mixed C-L / T-E requests

Disable keepalive on the connection if we received both Content-Length and
chunked Transfer-Encoding in the request, to avoid confusion with front
intermediaries and potential further request/response splitting.

This is what we do already for mod_proxy backend connections in the same case.

While at it, replace draft httpbis links with final RFC7230's.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1879373 13f79535-47bb-0310-9956-ffa450edef68