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/sched/sched_setscheduler.c

@@ -0,0 +1,8 @@
+#include <sched.h>
+#include "syscall.h"
+
+int sched_setscheduler(pid_t pid, int sched, const struct sched_param *param)
+{
+	static const struct sched_param def;
+	return syscall(SYS_sched_setscheduler, pid, sched, &def);
+}