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/environ/env_setenv.c

@@ -143,7 +143,7 @@ int setenv(FAR const char *name, FAR const char *value, int overwrite)
        * the environment buffer; this will happen below.
        */
 
-      (void)env_removevar(group, pvar);
+      env_removevar(group, pvar);
     }
 
   /* Get the size of the new name=value string.  The +2 is for the '=' and for

sched/environ/env_unsetenv.c

@@ -90,7 +90,7 @@ int unsetenv(FAR const char *name)
     {
       /* It does!  Remove the name=value pair from the environment. */
 
-      (void)env_removevar(group, pvar);
+      env_removevar(group, pvar);
 
       /* Reallocate the new environment buffer */