MFC 283613,287374:

MFC 283613,287374:
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.

git-svn-id: svn://svn.freebsd.org/base/stable/10@288387 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f