sched: expose nxsched_ set/get APIs as system calls

[pkarashchenko]

Summary

• expose nxsched_ set/get APIs as syscalls
• move corresponding sched_ and pthread_ set/get APIs to libc
• remove _SCHED_XXXX macro based interface
• reduce number of system calls
• use nxsched_ set/get APIs in kernel code

Impact

Should be none. Refactoring only

Testing

Pass CI