Make POSIX real-time priorities behave correctly

[vangyzen]

Summary

POSIX requires each real-time priority to behave like a separate run queue.
Only expose one POSIX priority per run queue, not per Unix priority.

Add a unit test to [try to] verify that each priority is truly unique.

While I'm here, directly include the necessary headers, fix spelling,
and remove a pointless cast.

For reference:
http://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html#tag_15_08_04