Revert "syscall, os: use pipe2 syscall on DragonflyBSD instead of pipe"

This reverts commit e6c15945.

Reason for revert: breaks the Dragonfly builders.

Fixes #27245

Change-Id: I2c147a5726aec28647f6ef5eb8f9db5efa3a9fd0
Reviewed-on: https://go-review.googlesource.com/131497


Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>