posix: work around "posix" systems without os.link available (issue4974) Some platforms (see bug, notably a terminal emulator on Android) ship with os.link removed from Python to try and cater to other tools that expect os.link to exist iff hardlinks are supported on that platform. As a workaround for this madness, include a fallback path for when we're on a "posix" platform but lack os.link.