net/dns: factor directManager out over an FS interface

This is preliminary work for using the directManager as
part of a wslManager on windows, where in addition to configuring
windows we'll use wsl.exe to edit the linux file system and modify the
system resolv.conf.

The pinholeFS is a little funky, but it's designed to work through
simple unix tools via wsl.exe without invoking bash. I would not have
thought it would stand on its own like this, but it turns out it's
useful for writing a test for the directManager.

Signed-off-by: David Crawshaw <crawshaw@tailscale.com>

net/dns/manager_freebsd.go

@@ -15,7 +15,7 @@ import (
 func NewOSConfigurator(logf logger.Logf, _ string) (OSConfigurator, error) {
 	bs, err := ioutil.ReadFile("/etc/resolv.conf")
 	if os.IsNotExist(err) {
-		return newDirectManager()
+		return newDirectManager(), nil
 	}
 	if err != nil {
 		return nil, fmt.Errorf("reading /etc/resolv.conf: %w", err)
@@ -25,6 +25,6 @@ func NewOSConfigurator(logf logger.Logf, _ string) (OSConfigurator, error) {
 	case "resolvconf":
 		return newResolvconfManager(logf)
 	default:
-		return newDirectManager()
+		return newDirectManager(), nil
 	}
 }