cpuset: Add compat shim to the sched_affinity functions

To allow to run a newer world on a pre-1400079 kernel a compat shims to
the sched_affinity functions has beed added.

Reported by: antoine
Tested by: antoine
Reviewed by: kib
Differential revision: https://reviews.freebsd.org/D38555
MFC after: 3 days

(cherry picked from commit cbc32e4c5e8427e0f1032d9d3aa5863dd1538c11)