net/netstat: start of new netstat package, with Windows for now

This will be used in a future change to do localhost connection
authentication. This lets us quickly map a localhost TCP connection to
a PID. (A future change will then map a pid to a user)

TODO: pull portlist's netstat code into this package. Then portlist
will be fast on Windows without requiring shelling out to netstat.exe.

net/netstat/netstat_noimpl.go

@@ -0,0 +1,11 @@
+// Copyright (c) 2020 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 !windows
+
+package netstat
+
+func get() (*Table, error) {
+	return nil, ErrNotImplemented
+}