Revert "tailcfg: remove v6-overlay debug option."

This reverts commit da4ec54756.

Since v6 got disabled for Windows nodes, I need the debug flag back
to figure out why it was broken.

Signed-off-by: David Anderson <danderson@tailscale.com>

tailcfg/tailcfg.go

@@ -634,6 +634,8 @@ type MapRequest struct {
 	// Current DebugFlags values are:
 	//     * "warn-ip-forwarding-off": client is trying to be a subnet
 	//       router but their IP forwarding is broken.
+	//     * "v6-overlay": IPv6 development flag to have control send
+	//       v6 node addrs
 	//     * "minimize-netmap": have control minimize the netmap, removing
 	//       peers that are unreachable per ACLS.
 	DebugFlags []string `json:",omitempty"`