kqueue: add some kn_knlist assertions around knlist_(add|remove)

[kevans]

Summary

We currently assert that kn_status is accurate, but there's more room
for error. Neither of these are very likely, but currently we'd blow up
in SLIST*() macros instead of providing more obvious diagnostics. It's
perhaps only worth testing these because knlist_remove() requires
getting logic across both f_attach() and f_detach() correct.