wgengine/magicsock: log more details of endpoints learned over disco

Also, don't try to use IPv6 LinkLocalUnicast addresses for now. Like endpoints
exchanged with control, we share them but don't yet use them.

Updates #1172

wgengine/magicsock/magicsock.go

@@ -1850,7 +1850,10 @@ func (c *Conn) handleDiscoMessage(msg []byte, src netaddr.IPPort) bool {
 			return true
 		}
 		if de != nil {
-			c.logf("magicsock: disco: %v<-%v (%v, %v)  got call-me-maybe", c.discoShort, de.discoShort, de.publicKey.ShortString(), derpStr(src.String()))
+			c.logf("magicsock: disco: %v<-%v (%v, %v)  got call-me-maybe, %d endpoints",
+				c.discoShort, de.discoShort,
+				de.publicKey.ShortString(), derpStr(src.String()),
+				len(dm.MyNumber))
 			go de.handleCallMeMaybe(dm)
 		}
 	}
@@ -3464,14 +3467,35 @@ func (de *discoEndpoint) handleCallMeMaybe(m *disco.CallMeMaybe) {
 	if de.isCallMeMaybeEP == nil {
 		de.isCallMeMaybeEP = map[netaddr.IPPort]bool{}
 	}
+	var newEPs []netaddr.IPPort
 	for _, ep := range m.MyNumber {
+		if ep.IP.Is6() && ep.IP.IsLinkLocalUnicast() {
+			// We send these out, but ignore them for now.
+			// TODO: teach the ping code to ping on all interfaces
+			// for these.
+			continue
+		}
 		de.isCallMeMaybeEP[ep] = true
 		if es, ok := de.endpointState[ep]; ok {
 			es.callMeMaybeTime = now
 		} else {
 			de.endpointState[ep] = &endpointState{callMeMaybeTime: now}
-		}
+			newEPs = append(newEPs, ep)
+		}
+	}
+	if len(newEPs) > 0 {
+		de.c.logf("magicsock: disco: call-me-maybe from %v %v added new endpoints: %v",
+			de.publicKey.ShortString(), de.discoShort,
+			logger.ArgWriter(func(w *bufio.Writer) {
+				for i, ep := range newEPs {
+					if i > 0 {
+						w.WriteString(", ")
+					}
+					w.WriteString(ep.String())
+				}
+			}))
 	}
+
 	// Delete any prior CalllMeMaybe endpoints that weren't included
 	// in this message.
 	for ep, want := range de.isCallMeMaybeEP {