kevent: Fix races between timer detach and kqtimer_proc_continue()

[markj]

Summary

- When detaching a knote, we need to double check the enqueued flag
  after acquiring the process lock, as kqtimer_proc_continue() may have
  toggled it.
- kqtimer_proc_continue() could in principle reschedule a stopped
  callout after filt_timerdetach() drains the callout.  So, we need to
  re-check.

Reported by: [e-mail]
Reported by: [e-mail]