Pull request: * all: move internal Go packages to internal/

Merge in DNS/adguard-home from 2234-move-to-internal to master

Squashed commit of the following:

commit d26a288cabeac86f9483fab307677b1027c78524
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Fri Oct 30 12:44:18 2020 +0300

    * all: move internal Go packages to internal/

    Closes #2234.

internal/dhcpd/helpers.go

@@ -0,0 +1,76 @@
+package dhcpd
+
+import (
+	"encoding/binary"
+	"fmt"
+	"net"
+
+	"github.com/AdguardTeam/golibs/log"
+	"github.com/joomcode/errorx"
+)
+
+func isTimeout(err error) bool {
+	operr, ok := err.(*net.OpError)
+	if !ok {
+		return false
+	}
+	return operr.Timeout()
+}
+
+// Get IPv4 address list
+func getIfaceIPv4(iface net.Interface) []net.IP {
+	addrs, err := iface.Addrs()
+	if err != nil {
+		return nil
+	}
+
+	var res []net.IP
+	for _, a := range addrs {
+		ipnet, ok := a.(*net.IPNet)
+		if !ok {
+			continue
+		}
+		if ipnet.IP.To4() != nil {
+			res = append(res, ipnet.IP.To4())
+		}
+	}
+	return res
+}
+
+func wrapErrPrint(err error, message string, args ...interface{}) error {
+	var errx error
+	if err == nil {
+		errx = fmt.Errorf(message, args...)
+	} else {
+		errx = errorx.Decorate(err, message, args...)
+	}
+	log.Println(errx.Error())
+	return errx
+}
+
+func parseIPv4(text string) (net.IP, error) {
+	result := net.ParseIP(text)
+	if result == nil {
+		return nil, fmt.Errorf("%s is not an IP address", text)
+	}
+	if result.To4() == nil {
+		return nil, fmt.Errorf("%s is not an IPv4 address", text)
+	}
+	return result.To4(), nil
+}
+
+// Return TRUE if subnet mask is correct (e.g. 255.255.255.0)
+func isValidSubnetMask(mask net.IP) bool {
+	var n uint32
+	n = binary.BigEndian.Uint32(mask)
+	for i := 0; i != 32; i++ {
+		if n == 0 {
+			break
+		}
+		if (n & 0x80000000) == 0 {
+			return false
+		}
+		n <<= 1
+	}
+	return true
+}