implement stub versions of sched_*

these actually work, but for now they prohibit actually setting
priority levels and report min/max priority as 0.

src/misc/sched_yield.c

@@ -1,10 +0,0 @@
-#include <sched.h>
-#include "syscall.h"
-#include "libc.h"
-
-int __yield()
-{
-	return syscall(SYS_sched_yield);
-}
-
-weak_alias(__yield, sched_yield);