gh-111178: fix UBSan failures in Modules/selectmodule.c

[picnixz]

This PR fixes the UBSan failures and addresses some minor cosmetic changes. PEP-7 changes were not applied since they could scramble the diff but other semantic changes affecting the signature of touched functions may have been done.

• Issue: UBSan: Calling a function through pointer to incorrect function type is undefined behavior #111178

[vstinner]

LGTM

[picnixz]

@vstinner I've observed that _select_exec was not declared static so I declared it static as well. I think it was an omission of not having it static. The commit message will be:

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

We also perform some cleanup by suppressing unused return values and renaming the
unused parameter in `METH_NOARGS` and getter/setter methods to `dummy` and `closure`
respectively for semantic purposes. We also explicitly declare `_select_exec` as a
`static` function.