io_uring support was default-disabled because of numerous kernel bugs
but those are all in the sqpoll (file i/o) parts of io_uring.
Batching of epoll_ctl calls through io_uring works fine, is a nice
optimization, and is therefore unconditionally enabled again.
The UV_USE_IO_URING environment variable now only affects sqpoll, and
only when the UV_LOOP_ENABLE_IO_URING_SQPOLL event loop flag is set.
Fixes: https://github.com/libuv/libuv/issues/4616
69bad8201b