mod_proxy_connect: Don't issue AH02447 on sockets hangups, let the read

mod_proxy_connect: Don't issue AH02447 on sockets hangups, let the read
determine whether it is a normal close or a real error. PR 57168.

Abort the client or backend connection on polling errors, but don't forcibly
abort the client side at the end (the core filters will do that otherwise
when necessary), so that lingering close and SSL shutdown can occur on normal
close.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1635645 13f79535-47bb-0310-9956-ffa450edef68