New directive ProxyIOBufferSize. Sets the size of the buffer used

when reading from a remote HTTP server in proxy.

Modify receive/send loop in proxy_http and proxy_ftp so that
should it be necessary, the remote server socket is closed before
transmitting the last buffer (set by ProxyIOBufferSize) to the
client. This prevents the backend server from being forced to hang
around while the last few bytes are transmitted to a slow client.
Fix the case where no error checking was performed on the final
brigade in the loop.
PR:
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git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@93811 13f79535-47bb-0310-9956-ffa450edef68