feature/portlist: pull portlist service porting into extension, use eventbus

And yay: tsnet (and thus k8s-operator etc) no longer depends on
portlist! And LocalBackend is smaller.

Removes 50 KB from the minimal binary.

Updates #12614

Change-Id: Iee04057053dc39305303e8bd1d9599db8368d926
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/node_backend.go

@@ -258,6 +258,12 @@ func (nb *nodeBackend) PeersForTest() []tailcfg.NodeView {
 	return ret
 }
 
+func (nb *nodeBackend) CollectServices() bool {
+	nb.mu.Lock()
+	defer nb.mu.Unlock()
+	return nb.netMap != nil && nb.netMap.CollectServices
+}
+
 // AppendMatchingPeers returns base with all peers that match pred appended.
 //
 // It acquires b.mu to read the netmap but releases it before calling pred.