OS X 10.6 doesn't do O_CLOEXEC.
OS X 10.7 does.

For now, always fall back to using syscall.CloseOnExec on darwin.

This can removed when 10.6 is old news, or if we find a
way to cheaply & reliably detect 10.6 vs 10.7 at runtime.

Fixes #2587

R=golang-dev, rsc, iant
CC=golang-dev
https://golang.org/cl/5500053