Fixes to the scheduler timeslice management.

One is related to the time distribution when a new child is created.
If the parent has one tick left, then both child and parent received
zero tick. When combined with
	current_irq_nest_count = 1
	voluntary_preempt = 0
values, this caused the scheduler from being invoked.

Second is related to the overall time distribution. When a thread
runs out of time, its new time slice is calculated by the below
formula:
	new_timeslice = (thread_prio * SCHED_TICKS) / total_prio
If we consider total_prio is equal to the sum of the priorities of
all the threads in the system, it imposes a problem of getting
zero tick. In the new scenario, total_prio is equal to the priority
types in the system so it is fixed. Every thread gets a timeslice
in proportion of their priorities. Thus, there is no risk of taking
zero tick.

include/l4/generic/scheduler.h

@@ -17,6 +17,7 @@
 #define TASK_PRIO_SERVER	6
 #define TASK_PRIO_NORMAL	4
 #define TASK_PRIO_LOW		2
+#define TASK_PRIO_TOTAL		30
 
 /* Ticks per second, try ticks = 1000 + timeslice = 1 for regressed preemption test. */
 #define SCHED_TICKS				100
@@ -25,7 +26,7 @@
  * A task can run continuously at this granularity,
  * even if it has a greater total time slice.
  */
-#define SCHED_GRANULARITY			SCHED_TICKS/10
+#define SCHED_GRANULARITY			SCHED_TICKS/50
 
 static inline struct ktcb *current_task(void)
 {