We map the POSIX semantics of `IPC_PRIVATE` by creating unnamed file
mapping objects on Windows. While that is not particularly useful for
ext/shmop, which is the only bundled extension which uses `shmget()`,
it may be useful for external extensions.
One of the conditions tested was expected to fail with "Permission denied",
but it doesn't when running as root. The memory segment was also leaked, hence
the split.