This option enables using sendfile() in netcat client mode
if a normal file (not stdin) is sent. If the option is enabled
but stdin is sent rather than a normal file, netcat falls back
to the combination of read() and write().
Using sendfile() provides a higher performance compared to
the combination of read() and write().
Also this option is useful for testing / debugging tcp_sendfile()
functionality of NuttX TCP/IP stack.
