unix: add *xattr functions on NetBSD

Reuse the functions added for FreeBSD.

Change-Id: Icf5bf65c82f91147659947f6ea296dd2555724a0
Reviewed-on: https://go-review.googlesource.com/128256
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

unix/syscall_netbsd_test.go

@@ -0,0 +1,20 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package unix_test
+
+// stringsFromByteSlice converts a sequence of attributes to a []string.
+// On NetBSD, each entry consists of a single byte containing the length
+// of the attribute name, followed by the attribute name.
+// The name is _not_ NULL-terminated.
+func stringsFromByteSlice(buf []byte) []string {
+	var result []string
+	i := 0
+	for i < len(buf) {
+		next := i + 1 + int(buf[i])
+		result = append(result, string(buf[i+1:next]))
+		i = next
+	}
+	return result
+}