cpuset(8): move to /bin/

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