[kernel] Change thread_sleep arg from time to duration

It seems more common to want to sleep for a duration than to sleep to a
specific time. Change the implementation to not make the process look up
the current time first. (Plus, there's no current syscall to do so)

definitions/objects/thread.def

@@ -18,6 +18,6 @@ object thread : object {
     }
 
     method sleep [static] {
-        param until uint64
+        param duration uint64
     }
 }