cherry-pick: 3157 excessive ptrs

Merge in DNS/adguard-home from 3157-excessive-ptrs to master

Updates #3157.

Squashed commit of the following:

commit 6803988240dca2f147bb80a5b3f78d7749d2fa14
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Tue Apr 19 14:50:01 2022 +0300

    aghnet: and again

commit 1a7f4d1dbc8fd4d3ae620349917526a75fa71b47
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Tue Apr 19 14:49:20 2022 +0300

    aghnet: docs again

commit d88da1fc7135f3cd03aff10b02d9957c8ffdfd30
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Tue Apr 19 14:47:36 2022 +0300

    aghnet: imp docs

commit c45dbc7800e882c6c4110aab640c32b03046f89a
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Tue Apr 19 14:41:19 2022 +0300

    aghnet: keep alphabetical order

commit b61781785d096ef43f60fb4f1905a4ed3cdf7c68
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Tue Apr 19 13:50:56 2022 +0300

    aghnet: imp code quality

commit 578dbd71ed2f2089c69343d7d4bf8bbc29150ace
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Tue Apr 12 17:02:38 2022 +0300

    aghnet: imp arp container

internal/aghnet/arpdb_bsd.go

@@ -0,0 +1,76 @@
+//go:build darwin || freebsd
+// +build darwin freebsd
+
+package aghnet
+
+import (
+	"bufio"
+	"net"
+	"strings"
+	"sync"
+
+	"github.com/AdguardTeam/golibs/log"
+	"github.com/AdguardTeam/golibs/netutil"
+)
+
+func newARPDB() (arp *cmdARPDB) {
+	return &cmdARPDB{
+		parse: parseArpA,
+		ns: &neighs{
+			mu: &sync.RWMutex{},
+			ns: make([]Neighbor, 0),
+		},
+		cmd: "arp",
+		// Use -n flag to avoid resolving the hostnames of the neighbors.  By
+		// default ARP attempts to resolve the hostnames via DNS.  See man 8
+		// arp.
+		//
+		// See also https://github.com/AdguardTeam/AdGuardHome/issues/3157.
+		args: []string{"-a", "-n"},
+	}
+}
+
+// parseArpA parses the output of the "arp -a -n" command on macOS and FreeBSD.
+// The expected input format:
+//
+//   host.name (192.168.0.1) at ff:ff:ff:ff:ff:ff on en0 ifscope [ethernet]
+//
+func parseArpA(sc *bufio.Scanner, lenHint int) (ns []Neighbor) {
+	ns = make([]Neighbor, 0, lenHint)
+	for sc.Scan() {
+		ln := sc.Text()
+
+		fields := strings.Fields(ln)
+		if len(fields) < 4 {
+			continue
+		}
+
+		n := Neighbor{}
+
+		if ipStr := fields[1]; len(ipStr) < 2 {
+			continue
+		} else if ip := net.ParseIP(ipStr[1 : len(ipStr)-1]); ip == nil {
+			continue
+		} else {
+			n.IP = ip
+		}
+
+		hwStr := fields[3]
+		if mac, err := net.ParseMAC(hwStr); err != nil {
+			continue
+		} else {
+			n.MAC = mac
+		}
+
+		host := fields[0]
+		if err := netutil.ValidateDomainName(host); err != nil {
+			log.Debug("parsing arp output: %s", err)
+		} else {
+			n.Name = host
+		}
+
+		ns = append(ns, n)
+	}
+
+	return ns
+}