ipn/ipnlocal, net/tsdial: make SOCKS/HTTP dials use ExitDNS

And simplify, unexport some tsdial/netstack stuff in the the process.

Fixes #3475

Change-Id: I186a5a5cbd8958e25c075b4676f7f6e70f3ff76e
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/tsdial/dohclient_test.go

@@ -0,0 +1,32 @@
+// 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 tsdial
+
+import (
+	"context"
+	"flag"
+	"net"
+	"testing"
+	"time"
+)
+
+var dohBase = flag.String("doh-base", "", "DoH base URL for manual DoH tests; e.g. \"http://100.68.82.120:47830/dns-query\"")
+
+func TestDoHResolve(t *testing.T) {
+	if *dohBase == "" {
+		t.Skip("skipping manual test without --doh-base= set")
+	}
+	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+	defer cancel()
+	var r net.Resolver
+	r.Dial = func(ctx context.Context, network, address string) (net.Conn, error) {
+		return &dohConn{ctx: ctx, baseURL: *dohBase}, nil
+	}
+	addrs, err := r.LookupIP(ctx, "ip4", "google.com.")
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Logf("Got: %q", addrs)
+}