Pull request: dhcpd: do not assume mac addrs of 6 bytes

Closes #2828.

Squashed commit of the following:

commit 26c6cf81c32469e1c4955aafb40490c29b4d1a99
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Tue Mar 30 17:43:53 2021 +0300

    dhcpd: do not assume mac addrs of 6 bytes

internal/aghnet/addr.go

@@ -0,0 +1,23 @@
+package aghnet
+
+import (
+	"fmt"
+	"net"
+
+	"github.com/AdguardTeam/AdGuardHome/internal/agherr"
+)
+
+// ValidateHardwareAddress returns an error if hwa is not a valid EUI-48,
+// EUI-64, or 20-octet InfiniBand link-layer address.
+func ValidateHardwareAddress(hwa net.HardwareAddr) (err error) {
+	defer agherr.Annotate("validating hardware address %q: %w", &err, hwa)
+
+	switch l := len(hwa); l {
+	case 0:
+		return agherr.Error("address is empty")
+	case 6, 8, 20:
+		return nil
+	default:
+		return fmt.Errorf("bad len: %d", l)
+	}
+}