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.

libs/libc/wqueue/work_queue.c

@@ -87,7 +87,7 @@
 
 static int work_qqueue(FAR struct usr_wqueue_s *wqueue,
                        FAR struct work_s *work, worker_t worker,
-                       FAR void *arg, systime_t delay)
+                       FAR void *arg, clock_t delay)
 {
   DEBUGASSERT(work != NULL);
 
@@ -157,7 +157,7 @@ static int work_qqueue(FAR struct usr_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)
 {
   if (qid == USRWORK)
     {

libs/libc/wqueue/work_usrthread.c

@@ -119,11 +119,11 @@ void work_process(FAR struct usr_wqueue_s *wqueue)
   volatile FAR struct work_s *work;
   worker_t  worker;
   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;
   int ret;
 
   /* Then process queued work.  Lock the work queue while we process items