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/syscall_aix.go

@@ -281,7 +281,7 @@ func sendfile(outfd int, infd int, offset *int64, count int) (written int, err e
 }
 
 //sys	getdirent(fd int, buf []byte) (n int, err error)
-func ReadDirent(fd int, buf []byte) (n int, err error) {
+func Getdents(fd int, buf []byte) (n int, err error) {
 	return getdirent(fd, buf)
 }