Use the cpuset API more consistently.

[jhb]

Summary

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.

Test Plan

I've been running pmcstat with this for my other pmcstat testing recently.

[jhb]

Any thoughts?

[emaste]

No objection from me