ipn: split LocalBackend off into new ipn/ipnlocal package

And move a couple other types down into leafier packages.

Now cmd/tailscale doesn't bring in netlink, magicsock, wgengine, etc.

Fixes #1181

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

types/preftype/netfiltermode.go

@@ -0,0 +1,30 @@
+// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package preftype is a leaf package containing types for various
+// preferences.
+package preftype
+
+// NetfilterMode is the firewall management mode to use when
+// programming the Linux network stack.
+type NetfilterMode int
+
+const (
+	NetfilterOff      NetfilterMode = iota // remove all tailscale netfilter state
+	NetfilterNoDivert                      // manage tailscale chains, but don't call them
+	NetfilterOn                            // manage tailscale chains and call them from main chains
+)
+
+func (m NetfilterMode) String() string {
+	switch m {
+	case NetfilterOff:
+		return "off"
+	case NetfilterNoDivert:
+		return "nodivert"
+	case NetfilterOn:
+		return "on"
+	default:
+		return "???"
+	}
+}