Sometimes netcat in server mode crashed with "psock_accept: ERROR: si_accept failed: -9".
And sometimes it crashed with "up_assert: Assertion failed at file:inet/inet_sockif.c line: 841 task: netcat".
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.
The buffer doesn't have the space for the terminating NUL.
Fix it by replacing it with fwrite, as there seems to be little reason
to use fprintf and NUL-terminated string in the first place.
