More sendfile work. Use new sendfile API in Apache, update Windows MPM

to begin using APR socket API.
Note:
sendfile on Unix side is broken. Need to detect for NULL hdtr. I'll do
it later this week if no one else steps up.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@84529 13f79535-47bb-0310-9956-ffa450edef68