Transceiver52M: Implement POWEROFF command

Add stop and restart capability through the POWEROFF and POWERON
commands. Calling stop causes receive streaming to cease, and I/O
threads to shutdown leaving only the control handling thread running.
Upon receiving a POWERON command, I/O threads and device streaming are
restarted.

Proper shutdown of the transceiver is now initiated by the destructor,
which calls the stop command internally to wind down and deallocate
threads.

Signed-off-by: Tom Tsou <tom@tsou.cc>
diff --git a/Transceiver52M/radioInterface.h b/Transceiver52M/radioInterface.h
index 877102f..b359cbd 100644
--- a/Transceiver52M/radioInterface.h
+++ b/Transceiver52M/radioInterface.h
@@ -78,7 +78,8 @@
 public:
 
   /** start the interface */
-  void start();
+  bool start();
+  bool stop();
 
   /** intialization */
   virtual bool init(int type);