net/netmon: move TailscaleInterfaceIndex out of netmon.State (#18428)

fixes tailscale/tailscale#18418

Both Serve and PeerAPI broke when we moved the TailscaleInterfaceName
into State, which is updated asynchronously and may not be
available when we configure the listeners.

This extracts the explicit interface name property from netmon.State
and adds as a static struct with getters that have proper error
handling.

The bug is only found in sandboxed Darwin clients, where we
need to know the Tailscale interface details in order to set up the
listeners correctly (they must bind to our interface explicitly to escape
the network sandboxing that is applied by NECP).

Currently set only sandboxed macOS and Plan9 set this but it will
also be useful on Windows to simplify interface filtering in netns.

Signed-off-by: Jonathan Nobels <jonathan@tailscale.com>

cmd/tailscaled/tailscaled.go

@@ -799,8 +799,9 @@ func tryEngine(logf logger.Logf, sys *tsd.System, name string) (onlyNetstack boo
 
 		if runtime.GOOS == "plan9" {
 			// TODO(bradfitz): why don't we do this on all platforms?
+			// TODO(barnstar): we do it on sandboxed darwin now
 			// We should. Doing it just on plan9 for now conservatively.
-			sys.NetMon.Get().SetTailscaleInterfaceName(devName)
+			netmon.SetTailscaleInterfaceProps(devName, 0)
 		}
 
 		r, err := router.New(logf, dev, sys.NetMon.Get(), sys.HealthTracker.Get(), sys.Bus.Get())