Enhance DNS failover reliability (#1637)

* Fix using wrong array index in log to avoid potential panic

* Increase gRPC connection timeout and add the timeout resolv.conf option

This makes sure the dns client is able to failover to a second
configured nameserver, if present. That is the case then when using the
dns `file` manager and a resolv.conf file generated for netbird.

* On file backup restore, remove the first NS if it's the netbird NS

* Bump dns mangager discovery message from debug to info to ease debugging

client/internal/dns/host_linux.go

@@ -65,7 +65,7 @@ func newHostManager(wgInterface string) (hostManager, error) {
 		return nil, err
 	}
 
-	log.Debugf("discovered mode is: %s", osManager)
+	log.Infof("System DNS manager discovered: %s", osManager)
 	return newHostManagerFromType(wgInterface, osManager)
 }