debugger: cleaner debugger aborting

Make the second "Abort Debugger" kill the gdb process directly
instead of relying on further communication.

Also fix some "unexpected" (but harmless) state transitions.

Change-Id: I0938ec76420fbd77ec4b7348819dd7f63763547f
Reviewed-by: hjk <qthjk@ovi.com>

src/plugins/debugger/gdb/gdbengine.h

@@ -242,6 +242,7 @@ private: ////////// General Interface //////////
     virtual void shutdownEngine();
     virtual void shutdownInferior();
     virtual void notifyInferiorSetupFailed();
+    virtual void abortDebugger();
 
     virtual bool acceptsDebuggerCommands() const;
     virtual void executeDebuggerCommand(const QString &command);