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.

include/nuttx/clock.h

@@ -43,8 +43,10 @@
 
 #include <nuttx/config.h>
 
+#include <sys/types.h>
 #include <stdint.h>
 #include <time.h>
+
 #include <nuttx/compiler.h>
 
 /****************************************************************************
@@ -215,6 +217,7 @@
 /****************************************************************************
  * Public Types
  ****************************************************************************/
+
 /* This structure is used to report CPU usage for a particular thread */
 
 #ifdef CONFIG_SCHED_CPULOAD
@@ -225,20 +228,15 @@ struct cpuload_s
 };
 #endif
 
-/* This type is the natural with of the system timer */
+/* This non-standard type used to hold relative clock ticks that may take
+ * negative values.  Because of its non-portable nature the type sclock_t
+ * should be used only within the OS proper and not by portable applications.
+ */
 
 #ifdef CONFIG_SYSTEM_TIME64
-typedef uint64_t systime_t;
+typedef int64_t sclock_t;
 #else
-typedef uint32_t systime_t;
-#endif
-
-/* This type used to hold relative ticks that may have negative value */
-
-#ifdef CONFIG_SYSTEM_TIME64
-typedef int64_t ssystime_t;
-#else
-typedef int32_t ssystime_t;
+typedef int32_t sclock_t;
 #endif
 
 /****************************************************************************
@@ -261,7 +259,7 @@ extern "C"
  */
 
 #ifdef __HAVE_KERNEL_GLOBALS
-EXTERN volatile systime_t g_system_timer;
+EXTERN volatile clock_t g_system_timer;
 
 #ifndef CONFIG_SYSTEM_TIME64
 #  define clock_systimer() g_system_timer
@@ -361,7 +359,7 @@ void clock_resynchronize(FAR struct timespec *rtc_diff);
  ****************************************************************************/
 
 #if !defined(__HAVE_KERNEL_GLOBALS) || defined(CONFIG_SYSTEM_TIME64)
-systime_t clock_systimer(void);
+clock_t clock_systimer(void);
 #endif
 
 /****************************************************************************

include/nuttx/net/sixlowpan.h

@@ -439,7 +439,7 @@ struct sixlowpan_reassbuf_s
    * be cancelled.
    */
 
-  systime_t rb_time;
+  clock_t rb_time;
 };
 
 /****************************************************************************

include/nuttx/sched.h

@@ -326,7 +326,7 @@ struct sporadic_s
   uint8_t   nrepls;                 /* Number of active replenishments          */
   uint32_t  repl_period;            /* Sporadic replenishment period            */
   uint32_t  budget;                 /* Sporadic execution budget period         */
-  systime_t eventtime;              /* Time thread suspended or [re-]started    */
+  clock_t   eventtime;              /* Time thread suspended or [re-]started    */
 
   /* This is the last interval timer activated */
 

include/nuttx/semaphore.h

@@ -321,7 +321,7 @@ int nxsem_timedwait(FAR sem_t *sem, FAR const struct timespec *abstime);
  *
  ****************************************************************************/
 
-int nxsem_tickwait(FAR sem_t *sem, systime_t start, uint32_t delay);
+int nxsem_tickwait(FAR sem_t *sem, clock_t start, uint32_t delay);
 
 /****************************************************************************
  * Name: nxsem_post

include/nuttx/wqueue.h

@@ -292,8 +292,8 @@ struct work_s
   struct dq_entry_s dq;  /* Implements a doubly linked list */
   worker_t  worker;      /* Work callback */
   FAR void *arg;         /* Callback argument */
-  systime_t qtime;       /* Time work queued */
-  systime_t delay;       /* Delay until work performed */
+  clock_t qtime;         /* Time work queued */
+  clock_t delay;         /* Delay until work performed */
 };
 
 /****************************************************************************
@@ -361,7 +361,7 @@ int work_usrstart(void);
  ****************************************************************************/
 
 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);
 
 /****************************************************************************
  * Name: work_cancel

include/sys/types.h

@@ -231,7 +231,12 @@ typedef int16_t      blksize_t;
 typedef unsigned int socklen_t;
 typedef uint16_t     sa_family_t;
 
-/* Used for system times in clock ticks (equivalent to systime_t) */
+/* Used for system times in clock ticks. This type is the natural width of
+ * the system timer.
+ *
+ * NOTE: The signed-ness of clock_t is not specified at OpenGroup.org.  An
+ * unsigned type is used to support the full range of the internal clock.
+ */
 
 #ifdef CONFIG_SYSTEM_TIME64
 typedef uint64_t     clock_t;