os: Implement Pipe for darwin, add smoke test.

Evidently when you read from pipes in Go, you have to adjust your slice length to reflect the number of bytes read.
Verified upstream behaves the same way.

src/syscall/syscall_libc.go

@@ -153,10 +153,6 @@ func Kill(pid int, sig Signal) (err error) {
 
 type SysProcAttr struct{}
 
-func Pipe2(p []int, flags int) (err error) {
-	return ENOSYS // TODO
-}
-
 // TODO
 type WaitStatus uint32