[PPRZ CENTER] Added some more verbose io watch callbacks.

Trying to investigate the issue of immediately closing processes in
paparazzi center on mac os x Mountain Lion and Mavericks.
[PPRZ CENTER] Different approach for io_watch signal handling.

This time we have only one watch reacting to OUT and HUP, wheras it is
ignoring OUT signal and running callback only for HUP.
[PPRZ CENTER] Capture more of the IO signals.

Trying to gather a bit more data on the signal behavior.
[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.
Merge pull request #592 from esden/pprz_center_osx_fix

Paparazzi Center on Mac OS X: fixes #290

The missing HUP signal was found in the simulated poll function in glib. The glib implementation is copied from glibc which does not implement HUP but only generates an IN event instead (Mac OS default behavior would be to generate HUP & IN events together). The solution I went for was to just check if the channel is empty on IN event, and call the HUP callback.