Adds new OS internal functions nxsig_sleep() and nxsig_usleep. These differ from the standard sleep() and usleep() in that (1) they don't cause cancellation points, and (2) don't set the errno variable (if applicable). All calls to sleep() and usleep() changed to calls to nxsig_sleep() and nxsig_usleep().

Squashed commit of the following:

    Change all calls to usleep() in the OS proper to calls to nxsig_usleep()

    sched/signal:  Add a new OS internal function nxsig_usleep() that is functionally equivalent to usleep() but does not cause a cancellaption point and does not modify the errno variable.

    sched/signal:  Add a new OS internal function nxsig_sleep() that is functionally equivalent to sleep() but does not cause a cancellaption point.

include/nuttx/signal.h

@@ -171,6 +171,70 @@ int nxsig_timedwait(FAR const sigset_t *set, FAR struct siginfo *info,
 int nxsig_nanosleep(FAR const struct timespec *rqtp,
                     FAR struct timespec *rmtp);
 
+/****************************************************************************
+ * Name: nxsig_sleep
+ *
+ * Description:
+ *   The nxsig_sleep() function will cause the calling thread to be
+ *   suspended from execution until either the number of real-time seconds
+ *   specified by the argument 'seconds' has elapsed or a signal is
+ *   delivered to the calling thread.
+ *
+ *   This is an internal OS interface.  It is functionally equivalent to
+ *   the standard sleep() application interface except that:
+ *
+ *   - It is not a cancellaction point, and
+ *   - There is no check that the action of the signal is to invoke a
+ *     signal-catching function or to terminate the process.
+ *
+ *   See the description of sleep() for additional information that is not
+ *   duplicated here.
+ *
+ * Parameters:
+ *   seconds - The number of seconds to sleep
+ *
+ * Returned Value:
+ *   If nxsig_sleep() returns because the requested time has elapsed, the
+ *   value returned will be zero (OK). If nxsig_sleep() returns because of
+ *   premature arousal due to delivery of a signal, the return value will
+ *   be the "unslept" amount (the requested time minus the time actually
+ *   slept) in seconds.
+ *
+ ****************************************************************************/
+
+unsigned int nxsig_sleep(unsigned int seconds);
+
+/****************************************************************************
+ * Name: nxsig_usleep
+ *
+ * Description:
+ *   The nxsig_usleep() function will cause the calling thread to be
+ *   suspended from execution until either the number of real-time
+ *   microseconds specified by the argument 'usec' has elapsed or a signal
+ *   is delivered to the calling thread. The suspension time may be longer
+ *   than requested due to the scheduling of other activity by the system.
+ *
+ *   This is an internal OS interface.  It is functionally equivalent to
+ *   the standard nxsig_usleep() application interface except that:
+ *
+ *   - It is not a cancellaction point, and
+ *   - It does not modify the errno value.
+ *
+ *   See the description of usleep() for additional information that is not
+ *   duplicated here.
+ *
+ * Parameters:
+ *   usec - the number of microseconds to wait.
+ *
+ * Returned Value:
+ *   This is an internal OS interface and should not be used by applications.
+ *   It follows the NuttX internal error return policy:  Zero (OK) is
+ *   returned on success.  A negated errno value is returned on failure.
+ *
+ ****************************************************************************/
+
+int nxsig_usleep(useconds_t usec);
+
 /****************************************************************************
  * Name: nxsig_notification
  *