Pull request: 3225 bsd dhcp

Merge in DNS/adguard-home from 3225-bsd-dhcp to master

Closes #3225.
Closes #3417.

Squashed commit of the following:

commit e7ea691824c7ebc8cafd8c9e206679346cbc8592
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Thu Aug 12 17:02:02 2021 +0300

    all: imp code, docs

commit 5b598fc18a9b69a0256569f4c691bb6a2193dfbd
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Thu Aug 12 16:28:12 2021 +0300

    all: mv logic, imp code, docs, log changes

commit e3e1577a668fe3e5c61d075c390e4bd7268181ba
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Thu Aug 12 14:15:10 2021 +0300

    dhcpd: imp checkother

commit 3cc8b058195c30a7ef0b7741ee8463270d9e47ff
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Wed Aug 11 13:20:18 2021 +0300

    all: imp bsd support

internal/aghnet/net_openbsd.go

@@ -0,0 +1,70 @@
+//go:build openbsd
+// +build openbsd
+
+package aghnet
+
+import (
+	"bufio"
+	"fmt"
+	"io"
+	"net"
+	"os"
+	"strings"
+
+	"github.com/AdguardTeam/AdGuardHome/internal/aghio"
+	"github.com/AdguardTeam/AdGuardHome/internal/aghos"
+	"github.com/AdguardTeam/golibs/errors"
+)
+
+func canBindPrivilegedPorts() (can bool, err error) {
+	return aghos.HaveAdminRights()
+}
+
+// maxCheckedFileSize is the maximum acceptable length of the /etc/hostname.*
+// files.
+const maxCheckedFileSize = 1024 * 1024
+
+func ifaceHasStaticIP(ifaceName string) (ok bool, err error) {
+	const filenameFmt = "/etc/hostname.%s"
+
+	filename := fmt.Sprintf(filenameFmt, ifaceName)
+	var f *os.File
+	if f, err = os.Open(filename); err != nil {
+		if errors.Is(err, os.ErrNotExist) {
+			err = nil
+		}
+
+		return false, err
+	}
+	defer func() { err = errors.WithDeferred(err, f.Close()) }()
+
+	var r io.Reader
+	r, err = aghio.LimitReader(f, maxCheckedFileSize)
+	if err != nil {
+		return false, err
+	}
+
+	return hostnameIfStaticConfig(r)
+}
+
+// hostnameIfStaticConfig checks if the interface is configured by
+// /etc/hostname.* to have a static IP.
+//
+// TODO(e.burkov):  The platform-dependent functions to check the static IP
+// address configured are rather similar.  Think about unifying common parts.
+func hostnameIfStaticConfig(r io.Reader) (has bool, err error) {
+	s := bufio.NewScanner(r)
+	for s.Scan() {
+		line := strings.TrimSpace(s.Text())
+		fields := strings.Fields(line)
+		if len(fields) >= 2 && fields[0] == "inet" && net.ParseIP(fields[1]) != nil {
+			return true, s.Err()
+		}
+	}
+
+	return false, s.Err()
+}
+
+func ifaceSetStaticIP(string) (err error) {
+	return aghos.Unsupported("setting static ip")
+}