Paparazzi Center on Mac OS X fix

This patch fixes issiue #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.

Overall the whole implementation in the pc_common.ml is bit kludgy and would be nice to streamline/clean up, but I rather change as little as possible for now to not introduce unexpected regressions while fixing this. :)

I tested the fix on current Ubuntu as well as Mavericks, and it "works for me"(TM)