cpuset(8): move to /bin/

Summary:
This change moves /usr/bin/cpuset to /bin/cpuset so it is more readily
available on hosts where the /usr partition might not be mounted at the
time / has been mounted.

Remove some complexity from rc.subr(8) since /bin is assumed to always
be present if/when / is mounted.

MFC after: 2 weeks
MFC with: 0661f93892a2, 271d552379af

Test Plan:

• Test out rc.subr change.
• Confirm that the installation logic does what's needed.

Relnotes: yes (moving cpuset may impact strict file permissions/mode checking)
Reviewers: kevans
Subscribers: imp
Differential Revision: https://reviews.freebsd.org/D40057