Improve comment on GHC.Event.Manager.closeFd_.

Making this comment stand alone and not reference the comment on closeFd.
Added missing wakeup in GHC.Event.Manager.closeFd_.

This missing wakeup may have affected the IO manager when used with the poll backend.
Removed GHC.Event.Manager.closeFd (no longer used).

closeFd_ is now used instead.
Rename GHC.Event.Manager.closeFd_ to GHC.Event.Manager.closeFd.
Refactored to allow closeFdWith to invoke registered callbacks.

This change allows a future modification to close the file descriptor after the file descriptors are removed from the backend, but before the callbacks are invoked.
closeFdWith closes the file after the file is unregistered from the backend and before invoking callbacks.
Tidy up closeFdWith.
Avoid conditional compilation to have more code statically checked for all platforms.
Replace conditional compilation with ordinary Bool and conditionals in GHC.Event.Manager.