Add SoftwareSerial::stopListening()

This allows one to explicitly stop a SoftwareSerial instance from
listening, without having to make another one listening.

libraries/SoftwareSerial/SoftwareSerial.cpp

@@ -195,6 +195,17 @@ bool SoftwareSerial::listen()
   return false;
 }
 
+// Stop listening. Returns true if we were actually listening.
+bool SoftwareSerial::stopListening()
+{
+  if (active_object == this)
+  {
+    active_object = NULL;
+    return true;
+  }
+  return false;
+}
+
 //
 // The receive routine called by the interrupt handler
 //

libraries/SoftwareSerial/SoftwareSerial.h

@@ -87,6 +87,7 @@ public:
   bool listen();
   void end();
   bool isListening() { return this == active_object; }
+  bool stopListening();
   bool overflow() { bool ret = _buffer_overflow; _buffer_overflow = false; return ret; }
   int peek();