tailcfg: remove most Debug fields, move bulk to nodeAttrs [capver 70]

Now a nodeAttr: ForceBackgroundSTUN, DERPRoute, TrimWGConfig,
DisableSubnetsIfPAC, DisableUPnP.

Kept support for, but also now a NodeAttr: RandomizeClientPort.

Removed: SetForceBackgroundSTUN, SetRandomizeClientPort (both never
used, sadly... never got around to them. But nodeAttrs are better
anyway), EnableSilentDisco (will be a nodeAttr later when that effort
resumes).

Updates #8923

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

wgengine/magicsock/derp.go

@@ -35,15 +35,14 @@ import (
 
 // useDerpRoute reports whether magicsock should enable the DERP
 // return path optimization (Issue 150).
+//
+// By default it's enabled, unless an environment variable
+// or control says to disable it.
 func useDerpRoute() bool {
 	if b, ok := debugUseDerpRoute().Get(); ok {
 		return b
 	}
-	ob := controlclient.DERPRouteFlag()
-	if v, ok := ob.Get(); ok {
-		return v
-	}
-	return true // as of 1.21.x
+	return !controlclient.DisableDRPO()
 }
 
 // derpRoute is a route entry for a public key, saying that a certain

wgengine/magicsock/magicsock.go

@@ -15,6 +15,7 @@ import (
 	"net/netip"
 	"reflect"
 	"runtime"
+	"slices"
 	"strconv"
 	"strings"
 	"sync"
@@ -1773,7 +1774,7 @@ func (c *Conn) SetNetworkMap(nm *netmap.NetworkMap) {
 	}
 
 	c.logf("[v1] magicsock: got updated network map; %d peers", len(nm.Peers))
-	heartbeatDisabled := debugEnableSilentDisco() || (c.netMap != nil && c.netMap.Debug != nil && c.netMap.Debug.EnableSilentDisco)
+	heartbeatDisabled := debugEnableSilentDisco()
 
 	// Set a maximum size for our set of endpoint ring buffers by assuming
 	// that a single large update is ~500 bytes, and that we want to not
@@ -2096,7 +2097,8 @@ func (c *Conn) shouldDoPeriodicReSTUNLocked() bool {
 			c.logf("magicsock: periodicReSTUN: idle for %v", idleFor.Round(time.Second))
 		}
 		if idleFor > sessionActiveTimeout {
-			if c.netMap != nil && c.netMap.Debug != nil && c.netMap.Debug.ForceBackgroundSTUN {
+			if c.netMap != nil && c.netMap.SelfNode != nil &&
+				slices.Contains(c.netMap.SelfNode.Capabilities, tailcfg.NodeAttrDebugForceBackgroundSTUN) {
 				// Overridden by control.
 				return true
 			}