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:	0661f93892, 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

libexec/rc/rc.subr

@@ -52,7 +52,7 @@ ID="/usr/bin/id"
 IDCMD="if [ -x $ID ]; then $ID -un; fi"
 PS="/bin/ps -ww"
 JID=0
-CPUSET="/usr/bin/cpuset"
+CPUSET="/bin/cpuset"
 
 # rc_service provides the path to the service script that we are executing.
 # This is not being set here in an execution context, necessarily, so it's
@@ -981,7 +981,7 @@ run_rc_command()
 	esac
 
 	_cpusetcmd=
-	if [ -n "$_cpuset" -a -x $CPUSET ]; then
+	if [ -n "$_cpuset" ]; then
 		_cpusetcmd="$CPUSET -l $_cpuset"
 	fi