Add Crystal::EventLoop#resume_all to replace #reopened

[ysbaddaden]

Allows to distinguish the "resume all waiters" from the actual "close" action. Only called on UNIX targets for now.

Extracted from #16128 and renamed from #before_close to #resume_all and replaces the now obsolete #reopened hook.

[ysbaddaden]

Changed state to DRAFT. We shall talk a bit on the API: do we want this general #resume_all method? do we want to keep #reopened and instead introduce a specific #before_close?

(I'm also wondering if we'll have to cancel the submitted async requests with io_uring... and I guess we'll have to at least cancel the read/write ones which means we'll need to track them, otherwise the fdlock references won't decrement until something happens on the fd 😭)

[ysbaddaden]

We probably want #before_close instead of #resume_all at least for io_uring: we could just shutdown a socket instead of having to remember which rings have a pending read/write operation.

See #10740 (comment)