cmd/cli: new flow for leaking queries to OS resolver

The current flow involves marking OS resolver as down, which is not
right at all, since ctrld depends on it for leaking queries.

This commits implements new flow, which ctrld will restore DNS settings
once leaking marked, allowing queries go to OS resolver until the
internet connection is established.

cmd/cli/prog.go

@@ -729,7 +729,7 @@ func (p *prog) dnsWatchdog(iface *net.Interface, nameservers []string, allIfaces
 			mainLog.Load().Debug().Msg("stop dns watchdog")
 			return
 		case <-ticker.C:
-			if p.leakingQuery.Load() || p.um.isChecking(upstreamOS) {
+			if p.leakingQuery.Load() {
 				return
 			}
 			if dnsChanged(iface, ns) {