packet minimization when doing sendfile...

When preparing to call iol_sendfile(), don't call ap_bflush() first.
Instead, pass the data ap_bflush() would have written (if any) to
iol_sendfile() as a header.  In practice, this prevents sending headers
separately from the first part of the file.
Submitted by:	Greg Ames
Reviewed by:	Jeff Trawick


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@85895 13f79535-47bb-0310-9956-ffa450edef68