tsconst, util/linuxfw, wgengine/router: move Linux fw consts to tsconst

Now cmd/derper doesn't depend on iptables, nftables, and netlink code :)

But this is really just a cleanup step I noticed on the way to making
tsnet applications able to not link all the OS router code which they
don't use.

Updates #17313

Change-Id: Ic7b4e04e3a9639fd198e9dbeb0f7bae22a4a47a9
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tsconst/tsconst.go

@@ -0,0 +1,15 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Package tsconst exports some constants used elsewhere in the
+// codebase.
+package tsconst
+
+// WintunInterfaceDesc is the description attached to Tailscale
+// interfaces on Windows. This is set by the WinTun driver.
+const WintunInterfaceDesc = "Tailscale Tunnel"
+const WintunInterfaceDesc0_14 = "Wintun Userspace Tunnel"
+
+// TailnetLockNotTrustedMsg is the error message used by network lock
+// and sniffed (via substring) out of an error sent over the network.
+const TailnetLockNotTrustedMsg = "this node is not trusted by network lock"