Merge changes from team/russell/http_filetxfer

Handle transferring large files from the built-in http server.  Previously, the
code attempted to malloc a block as large as the file itself.  Now it uses the
sendfile() system call so that the file isn't copied into userspace at all if
it is available.  Otherwise, it just uses a read/write of small chunks at a time.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@72701 65c4cc65-6c06-0410-ace0-fbb531ad65f3