epollexclusive: handle future rb_io_t deprecation
It looks like Ruby 3.3+ will hide rb_io_t internals and
get rid of the venerable `GetOpenFile' macro in favor of
`rb_io_descriptor'. `rb_io_descriptor' has been public
API since Ruby 3.1 and should be safe to use, and
is necessary for `raindrops' (a dependency of ours):
https://yhbt.net/raindrops-public/
20230609104805.39022-1-samuel.williams@oriontransfer.co.nz/
https://bugs.ruby-lang.org/issues/19057#note-17
We'll also avoid an unnecessary call to `rb_io_get_io' in
`get_readers' since `epio' (aka `self') can only be of
the Unicorn::Waiter IO subclass. However, we must still use
`rb_io_get_io' when handling non-self args in `prep_readers'.