Improve local pairing behavior when two computers both try to start the pairing process separately.

I was able to reproduce something very like this bug by starting
pairing separately on both computers under poor network conditions (ie,
weak wifi on my front porch). Neither computer showed an alert for the
PairReq messages it was seeing (intermittently) from the other.

So, I've made a new PairReq message that has not been seen before
always make the alert pop up, even if the assistant thinks it is
in the middle of its own pairing process (or even another pairing
process with a different box on the LAN).

(This shouldn't cause a rogue PairAck to disrupt a pairing process part
way through.)

Assistant/Pairing.hs

@@ -28,7 +28,7 @@ data PairStage
 	| PairAck
 	{- "I saw your PairAck; you can stop sending them." -}
 	| PairDone
-	deriving (Eq, Read, Show, Ord)
+	deriving (Eq, Read, Show, Ord, Enum)
 
 newtype PairMsg = PairMsg (Verifiable (PairStage, PairData, SomeAddr))
 	deriving (Eq, Read, Show)