core: implement connection assumption

Mark activation requests that contain connections to be assumed,
and use that to short-circuit various parts of the activation
process by not touching various device attributes, since they
are already set up.  Also ensure the device is not deactivated
when it initially becomes managed, because that would kill the
connection we are about to assume.

include/NetworkManager.h

@@ -357,6 +357,9 @@ typedef enum {
 	/* Carrier/link changed */
 	NM_DEVICE_STATE_REASON_CARRIER,
 
+	/* The device's existing connection was assumed */
+	NM_DEVICE_STATE_REASON_CONNECTION_ASSUMED,
+
 	/* Unused */
 	NM_DEVICE_STATE_REASON_LAST = 0xFFFF
 } NMDeviceStateReason;