socat: Exit when the file associated with an FD is destroyed

socat: Exit when the file associated with an FD is destroyed

In many scenarios the receipt of an EOF will coincide with the
teardown of the file which the FD is associated with by the
application on the other side of the file pipe. If a timeout (-t) is
used socat continues to poll for new data (read) and send any new data
(write) until the timeout. This can lead to input/output errors as the
FD exists still but the file it was openned on is gone, and no check
is done on writes.

To get around this a check is added after the EOF is received that the
file still exists, and if not we proceed to exit immediately. This
will avoid i/o errors and will also cause a quicker return as we don't
have to wait for the timeout.

The original author is Mark Asselstine <asselsm@gmail.com>.

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>