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_nonhosted.go

@@ -180,6 +180,9 @@ type SysProcAttr struct {
 func Getgroups() ([]int, error)         { return []int{1}, nil }
 func Gettimeofday(tv *Timeval) error    { return ENOSYS }
 func Kill(pid int, signum Signal) error { return ENOSYS }
+func Pipe2(p []int, flags int) (err error) {
+	return ENOSYS // TODO
+}
 func Sendfile(outfd int, infd int, offset *int64, count int) (written int, err error) {
 	return 0, ENOSYS
 }