cherry-pick: 3142 swap arp and rdns priority

Merge in DNS/adguard-home from 3142-fix-clients to master

Updates #3142.
Updates #3597.

Squashed commit of the following:

commit 4dcabedbfb1a4e4a0aaba588f708e4625442fce8
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Tue Mar 22 15:13:15 2022 +0300

    all: imp log of changes

commit 481088d05eecac1109daf378e0b4d5f6b2cf099b
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Tue Mar 22 14:36:44 2022 +0300

    all: swap arp and rdns priority

internal/dnsforward/dnsforward.go

@@ -314,7 +314,7 @@ func (s *Server) Exchange(ip net.IP) (host string, err error) {
 		StartTime: time.Now(),
 	}
 
-	resolver := s.internalProxy
+	var resolver *proxy.Proxy
 	if s.subnetDetector.IsLocallyServedNetwork(ip) {
 		if !s.conf.UsePrivateRDNS {
 			return "", nil
@@ -322,6 +322,8 @@ func (s *Server) Exchange(ip net.IP) (host string, err error) {
 
 		resolver = s.localResolvers
 		s.recDetector.add(*req)
+	} else {
+		resolver = s.internalProxy
 	}
 
 	if err = resolver.Resolve(ctx); err != nil {

internal/home/clients.go

@@ -56,8 +56,8 @@ type clientSource uint
 // Client sources.  The order determines the priority.
 const (
 	ClientSourceWHOIS clientSource = iota
-	ClientSourceRDNS
 	ClientSourceARP
+	ClientSourceRDNS
 	ClientSourceDHCP
 	ClientSourceHostsFile
 )
@@ -721,9 +721,7 @@ func (clients *clientsContainer) AddHost(ip net.IP, host string, src clientSourc
 	clients.lock.Lock()
 	defer clients.lock.Unlock()
 
-	ok = clients.addHostLocked(ip, host, src)
-
-	return ok, nil
+	return clients.addHostLocked(ip, host, src), nil
 }
 
 // addHostLocked adds a new IP-hostname pairing.  For internal use only.

internal/home/rdns.go

@@ -124,14 +124,12 @@ func (r *RDNS) workerLoop() {
 			log.Debug("rdns: resolving %q: %s", ip, err)
 
 			continue
-		}
-
-		if host == "" {
+		} else if host == "" {
 			continue
 		}
 
-		// Don't handle any errors since AddHost doesn't return non-nil
-		// errors for now.
+		// Don't handle any errors since AddHost doesn't return non-nil errors
+		// for now.
 		_, _ = r.clients.AddHost(ip, host, ClientSourceRDNS)
 	}
 }