cpuset(9): Add CPU_FOREACH_IS(SET|CLR) and modify consumers to use it

This implementation is faster and doesn't modify the cpuset, so it lets
us avoid some unnecessary copying as well. No functional change
intended.

This is a re-application of commit
9068f6ea697b1b28ad1326a4c7a9ba86f08b985e.

Reviewed by: cem, kib, jhb
Sponsored by: The FreeBSD Foundation

(cherry picked from commit de8554295b47475e758a573ab7418265f21fee7e)