os: reduce allocations in Readdir on unix

os: reduce allocations in Readdir on unix

Include syscall.Stat_t on unix to the
unexported fileStat structure rather than
accessing it though an interface.

Additionally add a benchmark for Readdir
(and Readdirnames).

Tested on linux, freebsd, netbsd, openbsd
darwin, solaris, does not touch windows
stuff. Does not change the API, as
discussed on golang-dev.

E.g. on linux/amd64 with a directory of 65 files:

benchmark              old ns/op     new ns/op     delta
BenchmarkReaddir-4     67774         66225         -2.29%

benchmark              old allocs     new allocs     delta
BenchmarkReaddir-4     334            269            -19.46%

benchmark              old bytes     new bytes     delta
BenchmarkReaddir-4     25208         24168         -4.13%

Change-Id: I44ef72a04ad7055523a980f29aa11122040ae8fe
Reviewed-on: https://go-review.googlesource.com/16423
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>