control/controlknobs, all: add plumbed Knobs type, not global variables

Previously two tsnet nodes in the same process couldn't have disjoint
sets of controlknob settings from control as both would overwrite each
other's global variables.

This plumbs a new controlknobs.Knobs type around everywhere and hangs
the knobs sent by control on that instead.

Updates #9351

Change-Id: I75338646d36813ed971b4ffad6f9a8b41ec91560
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tsconnect/wasm/wasm_js.go

@@ -102,6 +102,7 @@ func newIPN(jsConfig js.Value) map[string]any {
 	eng, err := wgengine.NewUserspaceEngine(logf, wgengine.Config{
 		Dialer:       dialer,
 		SetSubsystem: sys.Set,
+		ControlKnobs: sys.ControlKnobs(),
 	})
 	if err != nil {
 		log.Fatal(err)