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>

wgengine/magicsock/derp.go

@@ -18,7 +18,6 @@ import (
 	"time"
 
 	"github.com/tailscale/wireguard-go/conn"
-	"tailscale.com/control/controlclient"
 	"tailscale.com/derp"
 	"tailscale.com/derp/derphttp"
 	"tailscale.com/health"
@@ -38,11 +37,11 @@ import (
 //
 // By default it's enabled, unless an environment variable
 // or control says to disable it.
-func useDerpRoute() bool {
+func (c *Conn) useDerpRoute() bool {
 	if b, ok := debugUseDerpRoute().Get(); ok {
 		return b
 	}
-	return !controlclient.DisableDRPO()
+	return c.controlKnobs == nil || !c.controlKnobs.DisableDRPO.Load()
 }
 
 // derpRoute is a route entry for a public key, saying that a certain
@@ -294,7 +293,7 @@ func (c *Conn) derpWriteChanOfAddr(addr netip.AddrPort, peer key.NodePublic) cha
 	// perhaps peer's home is Frankfurt, but they dialed our home DERP
 	// node in SF to reach us, so we can reply to them using our
 	// SF connection rather than dialing Frankfurt. (Issue 150)
-	if !peer.IsZero() && useDerpRoute() {
+	if !peer.IsZero() && c.useDerpRoute() {
 		if r, ok := c.derpRoute[peer]; ok {
 			if ad, ok := c.activeDerp[r.derpID]; ok && ad.c == r.dc {
 				c.setPeerLastDerpLocked(peer, r.derpID, regionID)