A Python interface to sendfile(2) syscall.

sendfile(2) is a system call which provides a "zero-copy" way of
copying data from one file descriptor to another (a socket).

The phrase "zero-copy" refers to the fact that all of the copying of
data between the two descriptors is done entirely by the kernel, with
no copying of data into userspace buffers. This is particularly useful
when sending a file over a socket (e.g. FTP).
