control/controlknobs: move ForceBackgroundSTUN to controlknobs.Knobs

This is both more efficient (because the knobs' bool is only updated
whenever Node is changed, rarely) and also gets us one step closer to
removing a case of storing a netmap.NetworkMap in
magicsock. (eventually we want to phase out much of the use of that
type internally)

Updates #1909

Change-Id: I37e81789f94133175064fdc09984e4f3a431f1a1
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

control/controlclient/direct.go

@@ -1302,6 +1302,7 @@ func (ms *mapSession) setControlKnobsFromNodeAttrs(selfNodeAttrs []string) {
 		disableUPnP         bool
 		randomizeClientPort bool
 		oneCGNAT            opt.Bool
+		forceBackgroundSTUN bool
 	)
 	for _, attr := range selfNodeAttrs {
 		switch attr {
@@ -1317,6 +1318,8 @@ func (ms *mapSession) setControlKnobsFromNodeAttrs(selfNodeAttrs []string) {
 			oneCGNAT.Set(true)
 		case tailcfg.NodeAttrOneCGNATDisable:
 			oneCGNAT.Set(false)
+		case tailcfg.NodeAttrDebugForceBackgroundSTUN:
+			forceBackgroundSTUN = true
 		}
 	}
 	k.KeepFullWGConfig.Store(keepFullWG)
@@ -1324,6 +1327,7 @@ func (ms *mapSession) setControlKnobsFromNodeAttrs(selfNodeAttrs []string) {
 	k.DisableUPnP.Store(disableUPnP)
 	k.RandomizeClientPort.Store(randomizeClientPort)
 	k.OneCGNAT.Store(oneCGNAT)
+	k.ForceBackgroundSTUN.Store(forceBackgroundSTUN)
 }
 
 // ipForwardingBroken reports whether the system's IP forwarding is disabled

control/controlknobs/controlknobs.go

@@ -34,4 +34,8 @@ type Knobs struct {
 	// OneCGNAT is whether the the node should make one big CGNAT route
 	// in the OS rather than one /32 per peer.
 	OneCGNAT syncs.AtomicValue[opt.Bool]
+
+	// ForceBackgroundSTUN forces netcheck STUN queries to keep
+	// running in magicsock, even when idle.
+	ForceBackgroundSTUN atomic.Bool
 }