wgengine/magicsock: hook up discovery messages, upgrade to LAN works

Ping messages now go out somewhat regularly, pong replies are sent,
and pong replies are now partially handled enough to upgrade off DERP
to LAN.

CallMeMaybe packets are sent & received over DERP, but aren't yet
handled. That's next (and regular maintenance timers), and then WAN
should work.

Updates #483

disco/disco.go

@@ -28,6 +28,12 @@ import (
 	"inet.af/netaddr"
 )
 
+// Magic is the 6 byte header of all discovery messages.
+const Magic = "TS💬" // 6 bytes: 0x54 53 f0 9f 92 ac
+
+// NonceLen is the length of the nonces used by nacl secretboxes.
+const NonceLen = 24
+
 type MessageType byte
 
 const (