Extend cpuset_get/setaffinity() APIs

Add IRQ placement-only and ithread-only API variants. intr_event_bind
has been extended with sibling methods, as it has many more callsites in
existing code.

Reviewed by: kib@, adrian@ (earlier version)
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D10586