unix: merge implementation for ReadDirent

Provide common implementation for ReadDirent based on Getdents for
aix, *bsd and linux.

Add Getdents on aix as a wrapper around getdirent.

Keep the implementation for ReadDirent based on Getdirentries for
darwin and dragonfly.

Change-Id: I09094156a1452118ba48af4f14dfe44b1ab745d3
Reviewed-on: https://go-review.googlesource.com/c/sys/+/182321
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Benny Siegert <bsiegert@gmail.com>

unix/readdirent_getdents.go

@@ -0,0 +1,12 @@
+// Copyright 2019 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.
+
+// +build aix freebsd linux netbsd openbsd
+
+package unix
+
+// ReadDirent reads directory entries from fd and writes them into buf.
+func ReadDirent(fd int, buf []byte) (n int, err error) {
+	return Getdents(fd, buf)
+}