Make it possible to explicitly tell libuv to release any resources
it's still holding onto (memory, threads, file descriptors, etc.)
Before this commit, cleanup was performed in various destructors.
This commit centralizes the cleanup logic, enabling the addition of
`uv_library_shutdown()`, but maintains the current observable behavior
of cleaning up when libuv is unloaded by means of `dlclose(3)`.
Fixes: https://github.com/libuv/libuv/issues/2763
PR-URL: https://github.com/libuv/libuv/pull/2764
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Saúl Ibarra Corretgé <saghul@gmail.com>
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