wgengine/router: rely on events for deleted IP rules (#16744)

Adds the eventbus to the router subsystem.

The event is currently only used on linux.

Also includes facilities to inject events into the bus.

Updates #15160

Signed-off-by: Claus Lensbøl <claus@tailscale.com>

wgengine/router/router_openbsd.go

@@ -15,6 +15,7 @@ import (
 	"tailscale.com/health"
 	"tailscale.com/net/netmon"
 	"tailscale.com/types/logger"
+	"tailscale.com/util/eventbus"
 	"tailscale.com/util/set"
 )
 
@@ -31,7 +32,7 @@ type openbsdRouter struct {
 	routes  set.Set[netip.Prefix]
 }
 
-func newUserspaceRouter(logf logger.Logf, tundev tun.Device, netMon *netmon.Monitor, health *health.Tracker) (Router, error) {
+func newUserspaceRouter(logf logger.Logf, tundev tun.Device, netMon *netmon.Monitor, health *health.Tracker, bus *eventbus.Bus) (Router, error) {
 	tunname, err := tundev.Name()
 	if err != nil {
 		return nil, err