Fix wait loop and void cast (#24)

* Simplify EINTR/ECANCEL error handling

1. Add semaphore uninterruptible wait function
2 .Replace semaphore wait loop with a single uninterruptible wait
3. Replace all sem_xxx to nxsem_xxx

* Unify the void cast usage

1. Remove void cast for function because many place ignore the returned value witout cast
2. Replace void cast for variable with UNUSED macro

sched/clock/clock_initialize.c

@@ -185,7 +185,7 @@ static void clock_inittime(void)
     {
       struct timespec ts;
 
-      (void)clock_ticks2time((sclock_t)g_system_timer, &ts);
+      clock_ticks2time((sclock_t)g_system_timer, &ts);
 
       /* Adjust base time to hide initial timer ticks. */
 
@@ -336,7 +336,7 @@ void clock_resynchronize(FAR struct timespec *rtc_diff)
    * bias value that we need to use to correct the base time.
    */
 
-  (void)clock_systimespec(&bias);
+  clock_systimespec(&bias);
 
   /* Add the base time to this.  The base time is the time-of-day
    * setting.  When added to the elapsed time since the time-of-day

sched/clock/clock_settime.c

@@ -91,7 +91,7 @@ int clock_settime(clockid_t clock_id, FAR const struct timespec *tp)
        * bias value that we need to use to correct the base time.
        */
 
-      (void)clock_systimespec(&bias);
+      clock_systimespec(&bias);
 
       /* Save the new base time. */
 

sched/clock/clock_systimer.c

@@ -97,7 +97,7 @@ clock_t clock_systimer(void)
 
   /* Get the time from the platform specific hardware */
 
-  (void)clock_systimespec(&ts);
+  clock_systimespec(&ts);
 
   /* Convert to a 64-bit value in microseconds, then in clock tick units */
 
@@ -110,7 +110,7 @@ clock_t clock_systimer(void)
 
   /* Get the time from the platform specific hardware */
 
-  (void)clock_systimespec(&ts);
+  clock_systimespec(&ts);
 
   /* Convert to a 64- then a 32-bit value */