Pull request: * dhcpd: send secondary dns as well

Merge in DNS/adguard-home from 1708-secondary-dns to master

Updates #1708.

Squashed commit of the following:

commit 4529452e31131763f00c9c834cc95638f1a3d142
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Mon Nov 9 18:12:57 2020 +0300

    * dhcpd: send secondary dns as well

internal/dhcpd/dhcp_http.go

@@ -335,7 +335,7 @@ func (s *Server) handleDHCPFindActiveServer(w http.ResponseWriter, r *http.Reque
 	foundVal := "no"
 	if found4 {
 		foundVal = "yes"
-	} else if err != nil {
+	} else if err4 != nil {
 		foundVal = "error"
 		othSrv["error"] = err4.Error()
 	}