RFC: Listening for devices being added - udev vs. X11

[Martin Kelly]

> It looks like devd uses a config file, so xfsettingsd would need to add
> an entry to that. In any case, I don't have a BSD system to test on. How
> about if I #ifdef this to work only on Linux and if someone else with a
> BSD system has the time, then they could add the feature in? We could
> instead use X11, but then we are writing new code that we know will
> eventually die, which seems like bad design to me.

Even more directly, just check for the presence of udev instead of 
checking for Linux.