Flush stdin

[Dan Stromberg]

If I run the following in one tty:
    nc -l localhost 9000 | /tmp/z

...where /tmp/z has just:
    #!/usr/bin/python3

    import sys

    for line in sys.stdin.buffer:
        print(line)

And then run the following in another tty on the same computer:
    while read line; do echo $line; sleep 1; done < /etc/passwd | nc
localhost 9000

...then everything acts line buffered, or perhaps even character
buffered (the two are pretty indistinguishable in this test).  What I
see is my /etc/passwd file popping out of the nc -l side, one line at
a time, each line one second apart.

I suppose this suggests that it's the client that's sending TCP data
that is buffering.

That, or we're using two different versions of netcat (there are at
least two available).