- wsc module does not need nl80211 any longer, so remove it.
- Move wsc_init & wsc_exit declarations to iwd.h and remove wsc.h
- re-arrange how wsc_init & wsc_exit is called inside main.c.
Move the calling of the *_shutdown functions from the signal handler to
a new public function, and use that function inside the DBus disconnect
handler to make sure resources are cleanly released.