use sendfile on linux

[briot]

On linux at least (other unixes seem to have similar system calls), the most efficient way to send files is to use the sendfile() system call (rather than read the file, then write the bytes). This significantly reduces the number of system calls, and because sendfile can most of the time use Direct Memory Access, it also avoids extra copies of the file.

It would be nice if AWS could take advantage of that.