cmd/tailscale: fix default for tailscale set --accept-routes

The default values for `tailscale up` and `tailscale set` are supposed
to agree for all common flags. But they don’t for `--accept-routes`
on Windows and from the Mac OS App Store, because `tailscale up`
computes this value based on the operating system:

    user@host:~$ tailscale up --help 2>&1 | grep -A1 accept-routes
      --accept-dns, --accept-dns=false
            accept DNS configuration from the admin panel (default true)
    user@host:~$ tailscale set --help 2>&1 | grep -A1 accept-routes
      --accept-dns, --accept-dns=false
            accept DNS configuration from the admin panel

Luckily, `tailscale set` uses `ipn.MaskedPrefs`, so the default values
don’t logically matter. But someone will get the wrong idea if they
trust the `tailscale set --help` documentation.

In addition, `ipn.Prefs.RouteAll` defaults to true so it disagrees
with both of the flags above.

This patch makes `--accept-routes` use the same logic for in both
commands by hoisting the logic that was buried in `cmd/tailscale/cli`
to `ipn.Prefs.DefaultRouteAll`. Then, all three of defaults can agree.

Fixes: #15319

Signed-off-by: Simon Law <sfllaw@sfllaw.ca>

cmd/tailscale/cli/up.go

@@ -39,7 +39,6 @@ import (
 	"tailscale.com/types/preftype"
 	"tailscale.com/types/views"
 	"tailscale.com/util/dnsname"
-	"tailscale.com/version"
 	"tailscale.com/version/distro"
 )
 
@@ -79,14 +78,8 @@ func effectiveGOOS() string {
 // acceptRouteDefault returns the CLI's default value of --accept-routes as
 // a function of the platform it's running on.
 func acceptRouteDefault(goos string) bool {
-	switch goos {
-	case "windows":
-		return true
-	case "darwin":
-		return version.IsSandboxedMacOS()
-	default:
-		return false
-	}
+	var p *ipn.Prefs
+	return p.DefaultRouteAll(goos)
 }
 
 var upFlagSet = newUpFlagSet(effectiveGOOS(), &upArgsGlobal, "up")