router_linux: fix behaviour when switching --netfilter-mode.

On startup, and when switching into =off and =nodivert, we were
deleting netfilter rules even if we weren't the ones that added them.

In order to avoid interfering with rules added by the sysadmin, we have
to be sure to delete rules only in the case that we added them in the
first place.

Signed-off-by: Avery Pennarun <apenwarr@tailscale.com>

cmd/tailscale/tailscale.go

@@ -212,10 +212,10 @@ func runUp(ctx context.Context, args []string) error {
 			prefs.NetfilterMode = router.NetfilterOn
 		case "nodivert":
 			prefs.NetfilterMode = router.NetfilterNoDivert
-			warning("netfilter in nodivert mode, you must add calls to Tailscale netfilter chains manually")
+			warning("netfilter=nodivert; add iptables calls to ts-* chains manually.")
 		case "off":
 			prefs.NetfilterMode = router.NetfilterOff
-			warning("netfilter management disabled, you must write a secure packet filter yourself")
+			warning("netfilter=off; configure iptables yourself.")
 		default:
 			log.Fatalf("invalid value --netfilter-mode: %q", upArgs.netfilterMode)
 		}