daemon: use GLib signal handling

Make use of GLib signal handling instead of signalfd.  This has several
advantages:

 - signalfd is not well-defined in processes where you do not control
   the creation of all threads (and in particular their signal mask).
   GLib implicitly creates threads, so you should not use signalfd here.

 - signalfd is not portable which prevents dleyna-renderer from building
   on non-Linux systems (which it otherwise would)

 - using the GLib API gives a substantial reduction in complicated code

Signed-off-by: Ryan Lortie <desrt@desrt.ca>