ncat: perform half-duplex shutdown upon EOF

[James Stanley <james () incoherency co uk>]

I have submitted a pull request on github: 
https://github.com/nmap/nmap/pull/2510

The summary is:

Previously, |netexec()| would cease all communication as soon as it read EOF
from either the child proces *or* the remote side. This meant (for example)
when the other end of the TCP socket calls |shutdown(fd, SHUT_WR)|, the 
child
process doesn't get a chance to send any more response, drain its 
buffers, etc.

The new behaviour is to propagate EOF onwards from the child process to the
remote, and vice versa, and keep a flag variable for each side to say 
whether
we still want to be reading from that side.

------------------------------------------------------------------------

With the new version, if you do something like:

|server$ ncat -l 10000 -e /bin/cat client$ ncat localhost 10000 <file1 
>file2 |

Then it will now do what you expect: copy |file1| to |file2|. Previously 
ncat would stop reading from |/bin/cat| as soon as it got the 
half-duplex shutdown from the remote side, which means unless |cat|'s 
buffer is empty at the time ncat receives the shutdown, |file2| ends up 
shorter than |file1| because of the data stuck inside |cat| that never 
got transmitted over the network. Hope that makes sense.

Regards,

James Stanley

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