hubs: kqueue: initial implementation found in my repo
hubs: kqueue: enable kqueue hub; avoid modifying read-only attributes
hubs: kqueue: squashed fixes

* try and recover from bad file descriptor error after fork
* remove some stuff in kqueue hub that was added by accident
* skip test_closure test with kqueue hub
  kqueue hub cannot detect closures of file descriptors
* simplify _reinit_kqueue()