Patching disconnect to stop the print_thread

By setting printing to false and not restarting the sender thread when the print_thread exits in the disconnect method this should allow printcore to disconnect even if the print thread is running.

In particular this makes it possible to ctrl+c out of prontserve durring an m109 where it previously errored out and hung with the following error:

Can't read from printer (disconnected?) (SerialException): device reports readiness to read but returned no data (device disconnected?)

printcore.py

@@ -103,6 +103,9 @@ class printcore():
                 self.read_thread.join()
                 self.read_thread = None
             self._stop_sender()
+            if self.print_thread:
+                self.printing = False
+                self.print_thread.join()
             try:
                 self.printer.close()
             except socket.error:
@@ -426,7 +429,7 @@ class printcore():
             traceback.print_exc()
         finally:
             self.print_thread = None
-            self._start_sender()
+            if self.stop_send_thread == False: self._start_sender()
 
     #now only "pause" is implemented as host command
     def processHostCommand(self, command):