Implement shm_open for _posixshmem. Fixes oracle/graalpython#502

[ztawfick-ux]

Summary

_posixshmem was only a skeleton (a no-op shm_unlink, no shm_open), so multiprocessing.shared_memory.SharedMemory failed with AttributeError: module '_posixshmem' has no attribute 'shm_open'. This implements real POSIX shared memory end to end, so shared-memory IPC works on GraalPy without user code changes.

Fixes #502.

Changes

• Native helpers (python-libposix/src/posix.c): add call_shm_open / call_shm_unlink with errno capture.
• POSIX layer: add shmOpen / shmUnlink to PosixSupportLibrary with implementations in all backends — NativePosixSupport (NFI downcalls), plus Emulated/Logging/PreInit wrappers.
• Module builtins (PosixShMemModuleBuiltins): implement shm_open(path, flags, mode=0o777) returning the fd and a real shm_unlink(path), using the shared PathConversionNode and raising the correct OSError subclasses (FileExistsError / FileNotFoundError), mirroring os.open.
• stdlib (multiprocessing/resource_tracker.py): restore the upstream shared_memory → _posixshmem.shm_unlink cleanup registration (previously commented out).
• Build (suite.py): link librt (-lrt) on Linux so the symbols resolve on older glibc/musl.

Notes

• Works on the native POSIX backend. On the emulated (pure-Java) backend, shared memory isn't available and raises OSError, consistent with the existing sem_* behavior.

Testing

• New SharedMemoryTest in test_multiprocessing.py (create/write/read across handles, missing-segment and duplicate-create errors).

[msimacek]

Thank you for the contribution

[ztawfick-ux]

I don't think the CI failures were due to my code changes. How can I verify this?