gh-111178: fix UBSan failures in `Modules/selectmodule.c` (GH-129792)

gh-111178: fix UBSan failures in `Modules/selectmodule.c` (GH-129792)

Fix some UBSan failures for `pollObject`, `devpollObject`, `pyEpoll_Object` as well as
for `kqueue_event_Object`, `kqueue_queue_Object` and `kqueue_tracking_after_fork`.

Suppress unused return values.

Rename the unused parameter in `METH_NOARGS` and getter/setter methods to
`dummy` and `closure` respectively for semantic purposes.

Explicitly declare `_select_exec` as a `static` function.