posix: sched: Implement set APIs for scheduling parameters

Implement `sched_setparam()` and `sched_setscheduler()` POSIX APIs
as a part of PSE53 `_POSIX_PRIORITY_SCHEDULING` option group.
Both functions are actually placeholders and just return `ENOSYS`
since Zephyr does not yet support processes or process scheduling.

signed-off-by: Gaetan Perrot <gaetanperrotpro@gmail.com>