Thread creation privilege for realtime group

With the mac_priority(4) realtime policy active, users and processes in
the realtime group may promote existing threads and processes to
realtime scheduling priority. Extend the privileges granted to
PRIV_SCHED_SETPOLICY which allows explicit creation of new realtime
threads.

One use case of this is when the pthread scheduling policy is set to
SCHED_RR or SCHED_FIFO via pthread_attr_setschedpolicy(...) before
calling pthread_create(...). I ran into this when testing audio software
with realtime threads, particularly audio/ardour6.

MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D33393