1. Remove aio entry from lists earlier in aio_free_entry,

1. Remove aio entry from lists earlier in aio_free_entry,
   so other threads can not see it if we unlock the proc
   lock (this can happen in knlist_delete).  Don't do wakeup,
   it is not necessary.

2. Decrease kaio_buffer_count in biohelper rather than
   doing it in aio_bio_done_notify.

3. In aio_bio_done_notify, don't send notification if KAIO_RUNDOWN
   was set, because the process is already in single thread mode.

4. Use assignment to initialize aiothreadflags.

5. AIOCBLIST_RUNDOWN is not useful, axe the code using it.

6. use LIO_NOP instead of zero.