container: expose priority and SCHED_OTHER policy

The more explicit API removes the arbitrary limit preventing use of SCHED_OTHER (referred to as SCHED_NORMAL in the kernel). This change also exposes priority value to set.

Signed-off-by: Ophestra <cat@gensokyo.uk>

container/syscall.go

@@ -63,12 +63,12 @@ type schedParam struct {
 //
 // [very subtle to use correctly]: https://www.openwall.com/lists/musl/2016/03/01/4
 func schedSetscheduler(tid int, policy std.SchedPolicy, param *schedParam) error {
-	if r, _, errno := Syscall(
+	if _, _, errno := Syscall(
 		SYS_SCHED_SETSCHEDULER,
 		uintptr(tid),
 		uintptr(policy),
 		uintptr(unsafe.Pointer(param)),
-	); r < 0 {
+	); errno != 0 {
 		return errno
 	}
 	return nil