Use libc calls instead of raw syscalls on Darwin.

Change-Id: I0319ae61e3575e7f76b975b9325c9078ab3440d5
Reviewed-on: https://go-review.googlesource.com/c/149617Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>