Pull request: all: do not check local domains when dhcp srv is off

Updates #3028.

Squashed commit of the following:

commit 49d3ca5c9de0468ccb1792e9de263fd66e30d79c
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Thu Apr 29 15:35:32 2021 +0300

    all: do not check local domains when dhcp srv is off

internal/dnsforward/dns.go

@@ -249,6 +249,10 @@ func (s *Server) hostToIP(host string) (ip net.IP, ok bool) {
 //
 // TODO(a.garipov): Adapt to AAAA as well.
 func (s *Server) processInternalHosts(dctx *dnsContext) (rc resultCode) {
+	if !s.dhcpServer.Enabled() {
+		return resultCodeSuccess
+	}
+
 	req := dctx.proxyCtx.Req
 	q := req.Question[0]