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_openbsd.go

@@ -7,5 +7,5 @@ package dns
 import "tailscale.com/types/logger"
 
 func NewOSConfigurator(logger.Logf, string) (OSConfigurator, error) {
-	return newDirectManager()
+	return newDirectManager(), nil
 }