ipn/ipnlocal: use the fake peerapi listener as fallback if netstack available

The previous commit (1b89662eff) this for Android, but we can also use
this on any platform if we we would otherwise fail.

Change-Id: I4cd78b40e9e77fca5cc8e717dd48ac173101bed4
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/peerapi.go

@@ -436,8 +436,15 @@ func (s *peerAPIServer) listen(ip netaddr.IP, ifState *interfaces.State) (ln net
 			return ln, nil
 		}
 	}
-	// Fall back to random ephemeral port.
-	return lc.Listen(context.Background(), tcp4or6, net.JoinHostPort(ipStr, "0"))
+	// Fall back to some random ephemeral port.
+	ln, err = lc.Listen(context.Background(), tcp4or6, net.JoinHostPort(ipStr, "0"))
+
+	// And if we're on a platform with netstack (anything but iOS), then just fallback to netstack.
+	if err != nil && runtime.GOOS != "ios" {
+		s.b.logf("peerapi: failed to do peerAPI listen, harmless (netstack available) but error was: %v", err)
+		return newFakePeerAPIListener(ip), nil
+	}
+	return ln, err
 }
 
 type peerAPIListener struct {