[PPRZ CENTER] Fixed detection of child processes exitting on Mac OS.

We are now checking if the input channel is empty when receiving the IN
callback. If that is the case we assume that the child process exitted,
ond call the close process callback that normally is treggered by the
HUP callback.

Some background: On OS X glib is using it's own implementation of poll
that does not generate the HUP signal. (It would be possible to add this
functionality modelled after the SSH implementation of bsd_poll) The
glib implementation does generate IN signal when the pipe get's closed.
So we can just check if the input channel is empty and if that is the
case infer that the pipe got closed.