Fix #79566: Private SHM is not private on Windows

We map the POSIX semantics of `IPC_PRIVATE` by creating unnamed file
mapping objects on Windows. While that is not particularly useful

Fix #79566: Private SHM is not private on Windows

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.

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