kqueue: Only allow one thread to register event to a kqueue

kqueue: Only allow one thread to register event to a kqueue

This prevents various races on the registration path, if it
ever blocks (kq token will be released):
- Kqueue hash table creation; the malloc(M_WAIT) could block.
- Install a new event.  Holding the token protect the klist could block.

Since kqueue _should_ rarely be shared between threads, allowing one
thread to do the event registration to a kqueue can work well enough.