cmd/cli: preserve search domains when reverting resolv.conf

Fixes search domains not being preserved when the resolv.conf file is
reverted to its previous state. This ensures that important domain
search configuration is maintained during DNS configuration changes.

The search domains handling was missing in setResolvConf function,
which is responsible for restoring DNS settings.

cmd/cli/prog_linux.go

@@ -9,15 +9,12 @@ import (
 	"strings"
 
 	"github.com/kardianos/service"
-	"tailscale.com/control/controlknobs"
-	"tailscale.com/health"
 
-	"github.com/Control-D-Inc/ctrld/internal/dns"
 	"github.com/Control-D-Inc/ctrld/internal/router"
 )
 
 func init() {
-	if r, err := dns.NewOSConfigurator(func(format string, args ...any) {}, &health.Tracker{}, &controlknobs.Knobs{}, "lo"); err == nil {
+	if r, err := newLoopbackOSConfigurator(); err == nil {
 		useSystemdResolved = r.Mode() == "systemd-resolved"
 	}
 	// Disable quic-go's ECN support by default, see https://github.com/quic-go/quic-go/issues/3911