This code should be a noop today. But I did not feel comfortable about

  This code should be a noop today.  But I did not feel comfortable about
  reusing/recopying each bucket -after- inserting it into the brigade.
  Also, copying bucket c from b from a seemed obtuse.

  This patch creates the single bucket, makes a copy of it for each
  sendfile chunk we are about to add to the brigade, and finally consumes
  that initial bucket as the final file bucket of the brigade.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@93534 13f79535-47bb-0310-9956-ffa450edef68