Use the cpuset API more consistently:

• Fetch the root set from cpuset_getaffinity() instead of assuming all CPUs from 0 to hw.ncpu are the root set.
• Use CPU_SETSIZE and CPU_FFS.
• The original notion of halted CPUs the manpage and code refers to is gone. Use the term "available" instead.

Differential Revision: https://reviews.freebsd.org/D2491
Reviewed by: emaste
MFC after: 1 week