kernel - Make filters able to be marked MPSAFE

kernel - Make filters able to be marked MPSAFE

* Change struct filterops f_isfd field to f_flags, taking FILTEROP_ISFD and/or
  FILTEROP_MPSAFE.

* Convert all existing filter definitions to use new flags.

* Create filter_attach/detach/event wrapper functions for calling through the
  struct filterops vector that grab the MPLOCK as necessary.

* kern_event() uses kq->kq_count to determine whether or not to sleep,
  kqueue_scan() removes events from the TAILQ and can possibly sleep, releasing
  the global kq token, before updating kq->kq_count.