cmd/ctrld: optimizing set/reset DNS

Currently, when reset DNS, ctrld always find the net.Interface by
interface name. This may produce unexpected error because the interface
table may be cleared at the time ctrld is being stopped.

Instead, we can get the net interface only once, and use that interface
for restoring the DNS before shutting down.

While at it, also making logging message clearer.

cmd/ctrld/main.go

@@ -3,6 +3,7 @@ package main
 import (
 	"fmt"
 	"io"
+	"net"
 	"os"
 	"path/filepath"
 	"time"
@@ -35,6 +36,7 @@ var (
 
 	cdUID          string
 	iface          string
+	netIface       *net.Interface
 	ifaceStartStop string
 )