Various cleanups to the main function for AIO kernel processes:

• Pull the vmspace logic out into helper functions and reduce duplication. Operations on the vmspace are all isolated to vm_map.c, but it now exports a new 'vmspace_switch_aio' for use by AIO kernel processes.
• When an AIO kernel process wants to exit, break out of the main loop and perform cleanup after the loop end. This reduces a lot of indentation and allows cleanup to more closely mirror setup actions before the loop starts.
• Convert a DIAGNOSTIC to KASSERT().
• Replace mycp with more typical 'p'.

Reviewed by: kib
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D4990