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/sensors/xen1210.c

@@ -301,7 +301,7 @@ XEN1210_HANDLE xen1210_instantiate(FAR struct spi_dev_s *dev,
 
   /* Initialize the device state structure */
 
-  sem_init(&priv->exclsem, 0, 1);
+  nxsem_init(&priv->exclsem, 0, 1);
   priv->config = config;
 
   priv->spi = dev;