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_test.go

@@ -0,0 +1,57 @@
+package aghnet
+
+import (
+	"net"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestValidateHardwareAddress(t *testing.T) {
+	testCases := []struct {
+		name       string
+		wantErrMsg string
+		in         net.HardwareAddr
+	}{{
+		name:       "success_eui_48",
+		wantErrMsg: "",
+		in:         net.HardwareAddr{0x00, 0x01, 0x02, 0x03, 0x04, 0x05},
+	}, {
+		name:       "success_eui_64",
+		wantErrMsg: "",
+		in:         net.HardwareAddr{0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07},
+	}, {
+		name:       "success_infiniband",
+		wantErrMsg: "",
+		in: net.HardwareAddr{
+			0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07,
+			0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f,
+			0x10, 0x11, 0x12, 0x13,
+		},
+	}, {
+		name:       "error_nil",
+		wantErrMsg: `validating hardware address "": address is empty`,
+		in:         nil,
+	}, {
+		name:       "error_empty",
+		wantErrMsg: `validating hardware address "": address is empty`,
+		in:         net.HardwareAddr{},
+	}, {
+		name:       "error_bad",
+		wantErrMsg: `validating hardware address "00:01:02:03": bad len: 4`,
+		in:         net.HardwareAddr{0x00, 0x01, 0x02, 0x03},
+	}}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			err := ValidateHardwareAddress(tc.in)
+			if tc.wantErrMsg == "" {
+				assert.NoError(t, err)
+			} else {
+				require.Error(t, err)
+				assert.Equal(t, tc.wantErrMsg, err.Error())
+			}
+		})
+	}
+}