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>

net/portmapper/portmapper.go

@@ -18,6 +18,7 @@ import (
 	"time"
 
 	"go4.org/mem"
+	"tailscale.com/control/controlknobs"
 	"tailscale.com/net/interfaces"
 	"tailscale.com/net/netaddr"
 	"tailscale.com/net/neterror"
@@ -66,6 +67,7 @@ const trustServiceStillAvailableDuration = 10 * time.Minute
 type Client struct {
 	logf         logger.Logf
 	netMon       *netmon.Monitor // optional; nil means interfaces will be looked up on-demand
+	controlKnobs *controlknobs.Knobs
 	ipAndGateway func() (gw, ip netip.Addr, ok bool)
 	onChange     func() // or nil
 	debug        DebugKnobs
@@ -166,15 +168,19 @@ func (m *pmpMapping) Release(ctx context.Context) {
 // The debug argument allows configuring the behaviour of the portmapper for
 // debugging; if nil, a sensible set of defaults will be used.
 //
-// The optional onChange argument specifies a func to run in a new
-// goroutine whenever the port mapping status has changed. If nil,
-// it doesn't make a callback.
-func NewClient(logf logger.Logf, netMon *netmon.Monitor, debug *DebugKnobs, onChange func()) *Client {
+// The controlKnobs, if non-nil, specifies the control knobs from the control
+// plane that might disable portmapping.
+//
+// The optional onChange argument specifies a func to run in a new goroutine
+// whenever the port mapping status has changed. If nil, it doesn't make a
+// callback.
+func NewClient(logf logger.Logf, netMon *netmon.Monitor, debug *DebugKnobs, controlKnobs *controlknobs.Knobs, onChange func()) *Client {
 	ret := &Client{
 		logf:         logf,
 		netMon:       netMon,
 		ipAndGateway: interfaces.LikelyHomeRouterIP,
 		onChange:     onChange,
+		controlKnobs: controlKnobs,
 	}
 	if debug != nil {
 		ret.debug = *debug