Elimate use of the non-standard type systime_t and replace it the equivalent, standard type clock_t

Squashed commit of the following:

    sched:  Rename all use of system_t to clock_t.
    syscall:  Rename all use of system_t to clock_t.
    net:  Rename all use of system_t to clock_t.
    libs:  Rename all use of system_t to clock_t.
    fs:  Rename all use of system_t to clock_t.
    drivers:  Rename all use of system_t to clock_t.
    arch:  Rename all use of system_t to clock_t.
    include:  Remove definition of systime_t; rename all use of system_t to clock_t.

sched/wqueue/kwork_process.c

@@ -93,17 +93,17 @@
  *
  ****************************************************************************/
 
-void work_process(FAR struct kwork_wqueue_s *wqueue, systime_t period, int wndx)
+void work_process(FAR struct kwork_wqueue_s *wqueue, clock_t period, int wndx)
 {
   volatile FAR struct work_s *work;
   worker_t  worker;
   irqstate_t flags;
   FAR void *arg;
-  systime_t elapsed;
-  systime_t remaining;
-  systime_t stick;
-  systime_t ctick;
-  systime_t next;
+  clock_t elapsed;
+  clock_t remaining;
+  clock_t stick;
+  clock_t ctick;
+  clock_t next;
 
   /* Then process queued work.  We need to keep interrupts disabled while
    * we process items in the work list.

sched/wqueue/kwork_queue.c

@@ -88,7 +88,7 @@
 
 static void work_qqueue(FAR struct kwork_wqueue_s *wqueue,
                         FAR struct work_s *work, worker_t worker,
-                        FAR void *arg, systime_t delay)
+                        FAR void *arg, clock_t delay)
 {
   irqstate_t flags;
 
@@ -160,7 +160,7 @@ static void work_qqueue(FAR struct kwork_wqueue_s *wqueue,
  ****************************************************************************/
 
 int work_queue(int qid, FAR struct work_s *work, worker_t worker,
-               FAR void *arg, systime_t delay)
+               FAR void *arg, clock_t delay)
 {
   /* Queue the new work */
 

sched/wqueue/wqueue.h

@@ -74,7 +74,7 @@ struct kworker_s
 
 struct kwork_wqueue_s
 {
-  systime_t         delay;     /* Delay between polling cycles (ticks) */
+  clock_t           delay;     /* Delay between polling cycles (ticks) */
   struct dq_queue_s q;         /* The queue of pending work */
   struct kworker_s  worker[1]; /* Describes a worker thread */
 };
@@ -86,7 +86,7 @@ struct kwork_wqueue_s
 #ifdef CONFIG_SCHED_HPWORK
 struct hp_wqueue_s
 {
-  systime_t         delay;     /* Delay between polling cycles (ticks) */
+  clock_t           delay;     /* Delay between polling cycles (ticks) */
   struct dq_queue_s q;         /* The queue of pending work */
   struct kworker_s  worker[1]; /* Describes the single high priority worker */
 };
@@ -99,7 +99,7 @@ struct hp_wqueue_s
 #ifdef CONFIG_SCHED_LPWORK
 struct lp_wqueue_s
 {
-  systime_t         delay;  /* Delay between polling cycles (ticks) */
+  clock_t           delay;  /* Delay between polling cycles (ticks) */
   struct dq_queue_s q;      /* The queue of pending work */
 
   /* Describes each thread in the low priority queue's thread pool */
@@ -185,7 +185,7 @@ int work_lpstart(void);
  *
  ****************************************************************************/
 
-void work_process(FAR struct kwork_wqueue_s *wqueue, systime_t period, int wndx);
+void work_process(FAR struct kwork_wqueue_s *wqueue, clock_t period, int wndx);
 
 #endif /* CONFIG_SCHED_WORKQUEUE */
 #endif /* __SCHED_WQUEUE_WQUEUE_H */