MAC/priority module for realtime privilege group

This is a MAC policy module that grants scheduling privileges based on
group membership. Users or processes in the group realtime (gid 47) are
allowed to run threads and processes with realtime scheduling priority.
For timing-sensitive, low-latency software like audio/jack, running with
realtime priority helps to avoid stutter and gaps.

PR: 239125
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D33191