Squashed commit of the following:

libc/semaphore:  Add nxsem_getvalue() which is identical to sem_getvalue() except that it never modifies the errno variable.  Changed all references to sem_getvalue in the OS to nxsem_getvalue().

    sched/semaphore:  Rename all internal private functions from sem_xyz to nxsem_xyz.  The sem_ prefix is (will be) reserved only for the application semaphore interfaces.

    libc/semaphore:  Add nxsem_init() which is identical to sem_init() except that it never modifies the errno variable.  Changed all references to sem_init in the OS to nxsem_init().

    sched/semaphore:  Rename sem_tickwait() to nxsem_tickwait() so that it is clear this is an internal OS function.

    sched/semaphoate:  Rename sem_reset() to nxsem_reset() so that it is clear this is an internal OS function.

drivers/ioexpander/skeleton.c

@@ -760,7 +760,7 @@ FAR struct ioexpander_dev_s *skel_initialize(void)
 
 #endif
 
-  sem_init(&priv->exclsem, 0, 1);
+  nxsem_init(&priv->exclsem, 0, 1);
   return &priv->dev;
 }