mod_proxy_http: Use the "Connection: close" header for requests to

backends not recycling connections (disablereuse), including the default
reverse and forward proxies.

Submitted by: ylavic (and trawick for an old helper function)
Reviewed by: rjung, covener


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1673896 13f79535-47bb-0310-9956-ffa450edef68