posix: sched: add support for SCHED_OTHER

The `SCHED_OTHER` scheduling priority is mandatory as part of
POSIX. It must be numerically distinct from `SCHED_FIFO`,
`SCHED_RR`, and `SCHED_SPORADIC`, but is implementation-
defined and may behave identically to `SCHED_FIFO` or
`SCHED_RR`.

Signed-off-by: Chris Friedt <cfriedt@meta.com>