KQueue support for M:N threads

* Allows macOS users to use M:N threads (and technically FreeBSD, though it has not been verified on FreeBSD)

* Include sys/event.h header check for macros, and include sys/event.h when present

* Rename epoll_fd to more generic kq_fd (Kernel event Queue) for use by both epoll and kqueue

* MAP_STACK is not available on macOS so conditionall apply it to mmap flags

* Set fd to close on exec

* Log debug messages specific to kqueue and epoll on creation

* close_invalidate raises an error for the kqueue fd on child process fork. It's unclear rn if that's a bug, or if it's kqueue specific behavior

Use kq with rb_thread_wait_for_single_fd

* Only platforms with `USE_POLL` (linux) had changes applied to take advantage of kernel event queues. It needed to be applied to the `select` so that kqueue could be properly applied

* Clean up kqueue specific code and make sure only flags that were actually set are removed (or an error is raised)

* Also handle kevent specific errnos, since most don't apply from epoll to kqueue

* Use the more platform standard close-on-exec approach of `fcntl` and `FD_CLOEXEC`. The io-event gem uses `ioctl`, but fcntl seems to be the recommended choice. It is also what Go, Bun, and Libuv use

* We're making changes in this file anyways - may as well fix a couple spelling mistakes while here

Make sure FD_CLOEXEC carries over in dup

* Otherwise the kqueue descriptor should have FD_CLOEXEC, but doesn't and fails in assert_close_on_exec

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