Fix memory leak when shared libxml.dll is unloaded

When a multiple modules (process/plugins) all link to libxml2.dll they will in fact share a single loaded instance of it. It is unsafe for any of them to call xmlCleanupParser, as this would deinitialize the shared state and break others that might still have ongoing use.

However, on windows atexit is per-module (rather process-wide), so if used within libxml2 it is possible to register a clean up when all users are done and libxml2.dll is about to actually unload.

This allows multiple plugins to link with and share libxml2 without a premature cleanup if one is unloaded, while still cleaning up if all such callers are themselves unloaded.