aio rework

[Alexander Nasonov <alnsn%yandex.ru@localhost>]

Hi,

I recently noticed a flaw with the current implementation of aio, see
kern/45441. Basically, if you do aio_read and aio_write to one pipe
you might block a process's aio thread and thus block all aio activity
with current and future fildes of the process. All you can do in this
situation is to kill a process.

One possible solution is using separate threads for reading and writing.
It's a bit of work but it looks quite straitforward and I can do it at
my spare time.

Even with two threads, aio implementation is not optimal but I don't
have enough knowledge about NetBSD internals to make a better
implemention.

What do you guys think of my plan?

Thanks,
Alex