Squashed commit of the following:

This commit backs out most of commit b4747286b1.  That change was added because sem_wait() would sometimes cause cancellation points inappropriated.  But with these recent changes, nxsem_wait() is used instead and it is not a cancellation point.

    In the OS, all calls to sem_wait() changed to nxsem_wait().  nxsem_wait() does not return errors via errno so each place where nxsem_wait() is now called must not examine the errno variable.

    In all OS functions (not libraries), change sem_wait() to nxsem_wait().  This will prevent the OS from creating bogus cancellation points and from modifying the per-task errno variable.

    sched/semaphore:  Add the function nxsem_wait().  This is a new internal OS interface.  It is functionally equivalent to sem_wait() except that (1) it is not a cancellation point, and (2) it does not set the per-thread errno value on return.

net/usrsock/usrsock_dev.c

@@ -299,16 +299,21 @@ static uint8_t usrsockdev_get_xid(FAR struct usrsock_conn_s *conn)
 
 static void usrsockdev_semtake(FAR sem_t *sem)
 {
-  /* Take the semaphore (perhaps waiting) */
+  int ret;
 
-  while (sem_wait(sem) != 0)
+  do
     {
-      /* The only case that an error should occur here is if
-       * the wait was awakened by a signal.
+      /* Take the semaphore (perhaps waiting) */
+
+      ret = nxsem_wait(sem);
+
+      /* The only case that an error should occur here is if the wait was
+       * awakened by a signal.
        */
 
-      DEBUGASSERT(*get_errno_ptr() == EINTR);
+      DEBUGASSERT(ret == OK || ret == -EINTR);
     }
+  while (ret == -EINTR);
 }
 
 static void usrsockdev_semgive(FAR sem_t *sem)