core: generalize unavailable -> disconnected delayed transition

Instead of doing this in every device subclass, do it in the NMDevice
superclass.  nm_device_can_activate() already did the same logic that
each of the subclass device_state_changed() handlers were doing to
figure out whether they could do the transition from unavailable
to disconnected, so just use that in NMDevice and kill lots of code.

include/NetworkManager.h

@@ -362,6 +362,9 @@ typedef enum {
 	/* The device's existing connection was assumed */
 	NM_DEVICE_STATE_REASON_CONNECTION_ASSUMED,
 
+	/* The supplicant is now available */
+	NM_DEVICE_STATE_REASON_SUPPLICANT_AVAILABLE,
+
 	/* Unused */
 	NM_DEVICE_STATE_REASON_LAST = 0xFFFF
 } NMDeviceStateReason;