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>

control/controlclient/map.go

@@ -15,6 +15,7 @@ import (
 	"strconv"
 	"sync"
 
+	"tailscale.com/control/controlknobs"
 	"tailscale.com/envknob"
 	"tailscale.com/tailcfg"
 	"tailscale.com/tstime"
@@ -38,7 +39,8 @@ import (
 // one MapRequest).
 type mapSession struct {
 	// Immutable fields.
-	nu             NetmapUpdater // called on changes (in addition to the optional hooks below)
+	nu             NetmapUpdater       // called on changes (in addition to the optional hooks below)
+	controlKnobs   *controlknobs.Knobs // or nil
 	privateNodeKey key.NodePrivate
 	publicNodeKey  key.NodePublic
 	logf           logger.Logf
@@ -94,9 +96,10 @@ type mapSession struct {
 // Modify its optional fields on the returned value before use.
 //
 // It must have its Close method called to release resources.
-func newMapSession(privateNodeKey key.NodePrivate, nu NetmapUpdater) *mapSession {
+func newMapSession(privateNodeKey key.NodePrivate, nu NetmapUpdater, controlKnobs *controlknobs.Knobs) *mapSession {
 	ms := &mapSession{
 		nu:              nu,
+		controlKnobs:    controlKnobs,
 		privateNodeKey:  privateNodeKey,
 		publicNodeKey:   privateNodeKey.Public(),
 		lastDNSConfig:   new(tailcfg.DNSConfig),
@@ -184,7 +187,7 @@ func (ms *mapSession) HandleNonKeepAliveMapResponse(ctx context.Context, resp *t
 		if DevKnob.StripCaps() {
 			resp.Node.Capabilities = nil
 		}
-		setControlKnobsFromNodeAttrs(resp.Node.Capabilities)
+		ms.setControlKnobsFromNodeAttrs(resp.Node.Capabilities)
 	}
 
 	// Call Node.InitDisplayNames on any changed nodes.