AIO daemons have always been kernel processes to facilitate switching to

AIO daemons have always been kernel processes to facilitate switching to
user VM spaces while servicing jobs.  Update various comments and data
structures that refer to AIO daemons as threads to refer to processes
instead.

Reviewed by:	kib
Sponsored by:	Chelsio Communications
Differential Revision:	https://reviews.freebsd.org/D4999