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