Use proper convention for relative path linking

The change made in e835ee68e13361b841c983fa4a49dd6c19dcdec4 did not
follow the convention for relative path symlinks. Per the convention,
paths be prefixed with `${BINDIR}`, not `/bin/`. `${BINDIR}` can be
modified from the Makefile, on the command line, or in the
environment. This convention is the canonically correct way to do
things.

This follows the convention used in `bin/pkill/Makefile`,
`bin/timeout/Makefile`, etc.

MFC after: 1 week
MFC with: f05948d4e98d3abd0965a2994e9e42add6908ff3
MFC with: e835ee68e13361b841c983fa4a49dd6c19dcdec4
Requested by: jrtc27, kevans

Remove $FreeBSD$: one-line sh pattern

Remove /^\s*#[#!]?\s*\$FreeBSD\$.*$\n/

Link /usr/bin/cpuset to a relative path

This creates an appropriate symlink instead of a potentially incorrect
path pointing to the absolute path for cpuset(8) on the host.

MFC after: 2 weeks
MFC with: f05948d4e98d3abd0965a
Requested by: imp
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D41419

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