Squashed commit of the following:

sched/wqueue:  Modify high priority work queue to support multiple threads.

    sched/wqueue and libs/libc/wqueue:  workqueues don't need set global data to zero since .bss is cleared automatically.  Removing this unnecessary initialization also avoids the loss the work items queued before initialization.

include/nuttx/wqueue.h

@@ -65,6 +65,8 @@
  *   (which runs at the lowest of priority and may not be appropriate
  *   if memory reclamation is of high priority).  If CONFIG_SCHED_HPWORK
  *   is enabled, then the following options can also be used:
+ * CONFIG_SCHED_HPNTHREADS - The number of thread in the high-priority queue's
+ *   thread pool.  Default: 1
  * CONFIG_SCHED_HPWORKPRIORITY - The execution priority of the high-
  *   priority worker thread.  Default: 224
  * CONFIG_SCHED_HPWORKPERIOD - How often the worker thread checks for
@@ -146,6 +148,10 @@
 
 #ifdef CONFIG_SCHED_HPWORK
 
+#  ifndef CONFIG_SCHED_HPNTHREADS
+#    define CONFIG_SCHED_HPNTHREADS 1
+#  endif
+
 #  ifndef CONFIG_SCHED_HPWORKPRIORITY
 #    define CONFIG_SCHED_HPWORKPRIORITY 224
 #  endif