Start of netcheck package & including network state in Hostinfo.

* adds new packet "netcheck" to do the checking of UDP, IPv6, and
  nearest DERP server, and the Report type for all that (and more
  in the future, probably pulling in danderson's natprobe)
* new tailcfg.NetInfo type
* cmd/tailscale netcheck subcommand (tentative name, likely to
  change/move) to print out the netcheck.Report.

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

ipn/local_test.go

@@ -0,0 +1,25 @@
+// 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.
+
+package ipn
+
+import (
+	"flag"
+	"testing"
+
+	"tailscale.com/tailcfg"
+)
+
+var external = flag.Bool("external", false, "run external network tests")
+
+func TestPopulateNetworkConditions(t *testing.T) {
+	if !*external {
+		t.Skip("skipping network test without -external flag")
+	}
+	b := &LocalBackend{logf: t.Logf}
+	hi := new(tailcfg.Hostinfo)
+	b.populateNetworkConditions(hi)
+	t.Logf("Got: %+v", hi)
+
+}