libcrun: add crun_error_get_errno

[eriksjolund]

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Summary by Sourcery

Improve error handling by extracting and restoring original errno from libcrun error objects across cgroup setup, directory creation, and filesystem path operations.

Enhancements:

• Use crun_error_get_errno to retrieve and restore the original errno when cgroup directory creation fails.
• Preserve and reuse the saved errno in the directory creation helper before reporting errors.
• Restore errno from libcrun error objects in the masked path setup to ensure consistent error evaluation.