net/tstun: add start of Linux TAP support, with DHCP+ARP server

Still very much a prototype (hard-coded IPs, etc) but should be
non-invasive enough to submit at this point and iterate from here.

Updates #2589

Co-Author: David Crawshaw <crawshaw@tailscale.com>
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/tstun/tap_unsupported.go

@@ -0,0 +1,10 @@
+// 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.
+
+// +build !linux
+
+package tstun
+
+func (*Wrapper) handleTAPFrame([]byte) bool        { panic("unreachable") }
+func (*Wrapper) tapWrite([]byte, int) (int, error) { panic("unreachable") }