ipn/{ipnlocal,ipnstate}: add PeerStatus.ID stable ID to status --json output

Needed for the "up checker" to map back from exit node stable IDs (the
ipn.Prefs.ExitNodeID) back to an IP address in error messages.

But also previously requested so people can use it to then make API
calls. The upcoming "tailscale admin" subcommand will probably need it
too.

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

ipn/ipnlocal/local.go

@@ -326,6 +326,9 @@ func (b *LocalBackend) updateStatus(sb *ipnstate.StatusBuilder, extraLocked func
 		}
 	})
 	sb.MutateSelfStatus(func(ss *ipnstate.PeerStatus) {
+		if b.netMap != nil && b.netMap.SelfNode != nil {
+			ss.ID = b.netMap.SelfNode.StableID
+		}
 		for _, pln := range b.peerAPIListeners {
 			ss.PeerAPIURL = append(ss.PeerAPIURL, pln.urlStr)
 		}
@@ -365,6 +368,7 @@ func (b *LocalBackend) populatePeerStatusLocked(sb *ipnstate.StatusBuilder) {
 		}
 		sb.AddPeer(key.Public(p.Key), &ipnstate.PeerStatus{
 			InNetworkMap:       true,
+			ID:                 p.StableID,
 			UserID:             p.User,
 			TailAddrDeprecated: tailAddr4,
 			TailscaleIPs:       tailscaleIPs,