ipn/ipnserver, logpolicy: move Windows disk logging up earlier

This moves the Windows-only initialization of the filelogger into
logpolicy. Previously we only did it when babysitting the tailscaled
subprocess, but this meant that log messages from the service itself
never made it to disk. Examples that weren't logged to disk:

* logtail unable to dial out,
* DNS flush messages from the service
* svc.ChangeRequest messages (#3581)

This is basically the same fix as #3571 but staying in the Logf type,
and avoiding build-tagged file (which wasn't quite a goal, but
happened and seemed nice)

Fixes #3570

Co-authored-by: Aaron Klotz <aaron@tailscale.com>
Change-Id: Iacd80c4720b7218365ec80ae143339d030842702

logpolicy/logpolicy.go

@@ -13,6 +13,7 @@ import (
 	"crypto/tls"
 	"encoding/json"
 	"fmt"
+	"io"
 	"io/ioutil"
 	"log"
 	"net"
@@ -29,6 +30,7 @@ import (
 
 	"golang.org/x/term"
 	"tailscale.com/atomicfile"
+	"tailscale.com/log/filelogger"
 	"tailscale.com/logtail"
 	"tailscale.com/logtail/filch"
 	"tailscale.com/net/dnscache"
@@ -524,8 +526,20 @@ func New(collection string) *Policy {
 		}
 	}
 	lw := logtail.NewLogger(c, log.Printf)
+
+	var logOutput io.Writer = lw
+
+	if runtime.GOOS == "windows" && c.Collection == logtail.CollectionNode {
+		logID := newc.PublicID.String()
+		exe, _ := os.Executable()
+		if strings.EqualFold(filepath.Base(exe), "tailscaled.exe") {
+			diskLogf := filelogger.New("tailscale-service", logID, lw.Logf)
+			logOutput = logger.FuncWriter(diskLogf)
+		}
+	}
+
 	log.SetFlags(0) // other logflags are set on console, not here
-	log.SetOutput(lw)
+	log.SetOutput(logOutput)
 
 	log.Printf("Program starting: v%v, Go %v: %#v",
 		version.Long,