sched_setscheduler(2): Change realtime privilege check

Check for privilege PRIV_SCHED_SETPOLICY instead of PRIV_SCHED_SET, to
at least make it coherent with what is done at thread creation when
a realtime policy is requested, and have users authorized by
mac_priority(4) pass it.

This change is good enough in practice since it only allows 'root' (as
before) and mac_priority(4)'s authorized users in (the point of this
change), without other side effects. More changes in this area, to
generally ensure that all privilege checks are consistent, are going to
come as olce's priority revamp project lands.

(olce: Expanded the explanations.)

PR: 276962
Reported by: jbeich
Reviewed by: olce
Approved by: emaste (mentor)
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D43835