+ dns: respond to PTR requests for internal IP addresses from DHCP

Close #1682

Squashed commit of the following:

commit 2fad3544bf8853b1f8f19ad8b7bc8a490c96e533
Author: Simon Zolin <s.zolin@adguard.com>
Date:   Mon Jun 22 17:32:45 2020 +0300

    minor

commit 7c17992424702d95e6de91f30e8ae2dfcd8de257
Author: Simon Zolin <s.zolin@adguard.com>
Date:   Mon Jun 22 16:09:34 2020 +0300

    build

commit 16a52e11a015a97d3cbf30362482a4abd052192b
Merge: 7b6a73c8 2c47053c
Author: Simon Zolin <s.zolin@adguard.com>
Date:   Mon Jun 22 16:08:32 2020 +0300

    Merge remote-tracking branch 'origin/master' into 1682-dhcp-resolve

commit 7b6a73c84b5cb9a073a9dfb7d7bdecd22e1e1318
Author: Simon Zolin <s.zolin@adguard.com>
Date:   Mon Jun 22 16:01:34 2020 +0300

    tests

commit c2654abb2e5e7b7e3a04e4ddb8e1064b37613929
Author: Simon Zolin <s.zolin@adguard.com>
Date:   Mon Jun 1 15:15:13 2020 +0300

    + dnsforward: respond to PTR requests for internal IP addresses

    {[IP] => "host"} <- DNSforward <-(leases)-- DHCP

home/dns.go

@@ -61,7 +61,13 @@ func initDNSServer() error {
 	filterConf.HTTPRegister = httpRegister
 	Context.dnsFilter = dnsfilter.New(&filterConf, nil)
 
-	Context.dnsServer = dnsforward.NewServer(Context.dnsFilter, Context.stats, Context.queryLog)
+	p := dnsforward.DNSCreateParams{
+		DNSFilter:  Context.dnsFilter,
+		Stats:      Context.stats,
+		QueryLog:   Context.queryLog,
+		DHCPServer: Context.dhcpServer,
+	}
+	Context.dnsServer = dnsforward.NewServer(p)
 	dnsConfig := generateServerConfig()
 	err = Context.dnsServer.Prepare(&dnsConfig)
 	if err != nil {

home/whois_test.go

@@ -9,7 +9,7 @@ import (
 
 func prepareTestDNSServer() error {
 	config.DNS.Port = 1234
-	Context.dnsServer = dnsforward.NewServer(nil, nil, nil)
+	Context.dnsServer = dnsforward.NewServer(dnsforward.DNSCreateParams{})
 	conf := &dnsforward.ServerConfig{}
 	conf.UpstreamDNS = []string{"8.8.8.8"}
 	return Context.dnsServer.Prepare(conf)