allow IO#sendfile to send >4G files on 32-bit
allow IO#sendfile to send >4G files on 32-bit

Since IO#sendfile loops and retries anyways, we'll allow users
to send large files on 32-bit systems with one method call.
This makes life easier for people on 32-bit systems with 64-bit
off_t (the default for Ruby).

codeslinger/sendfile,codeslinger/sendfile