feature/featuretags: add a catch-all "Debug" feature flag

Saves 168 KB.

Updates #12614

Change-Id: Iaab3ae3efc6ddc7da39629ef13e5ec44976952ba
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/magicsock/debughttp.go

@@ -13,6 +13,8 @@ import (
 	"strings"
 	"time"
 
+	"tailscale.com/feature"
+	"tailscale.com/feature/buildfeatures"
 	"tailscale.com/tailcfg"
 	"tailscale.com/tstime/mono"
 	"tailscale.com/types/key"
@@ -24,6 +26,11 @@ import (
 // /debug/magicsock) or via peerapi to a peer that's owned by the same
 // user (so they can e.g. inspect their phones).
 func (c *Conn) ServeHTTPDebug(w http.ResponseWriter, r *http.Request) {
+	if !buildfeatures.HasDebug {
+		http.Error(w, feature.ErrUnavailable.Error(), http.StatusNotImplemented)
+		return
+	}
+
 	c.mu.Lock()
 	defer c.mu.Unlock()