net/dns: [win] add MagicDNS entries to etc/hosts

This works around the 2.3s delay in short name lookups when SNR is
enabled.
C:\Windows\System32\drivers\etc\hosts file. We only add known hosts that
match the search domains, and we populate the list in order of
Search Domains so that our matching algorithm mimics what Windows would
otherwise do itself if SNR was off.

Updates #1659

Signed-off-by: Maisem Ali <maisem@tailscale.com>

net/dns/manager_windows_test.go

@@ -5,6 +5,7 @@
 package dns
 
 import (
+	"bytes"
 	"context"
 	"fmt"
 	"math/rand"
@@ -21,6 +22,19 @@ import (
 
 const testGPRuleID = "{7B1B6151-84E6-41A3-8967-62F7F7B45687}"
 
+func TestHostFileNewLines(t *testing.T) {
+	in := []byte("#foo\r\n#bar\n#baz\n")
+	want := []byte("#foo\r\n#bar\r\n#baz\r\n")
+
+	got, err := setTailscaleHosts(in, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !bytes.Equal(got, want) {
+		t.Errorf("got %q, want %q\n", got, want)
+	}
+}
+
 func TestManagerWindowsLocal(t *testing.T) {
 	if !isWindows10OrBetter() || !winutil.IsCurrentProcessElevated() {
 		t.Skipf("test requires running as elevated user on Windows 10+")