ruby: fix deadlock on df exit due to BEGIN_UNLOAD + Core.Suspend()

plugins/ruby/README

@@ -116,7 +116,8 @@ To register ruby code to be run every graphic frame, use:
 To stop being called, use:
  df.onupdate_unregister handle
 
-The same mechanism is available for 'onstatechange'.
+The same mechanism is available for 'onstatechange', but the
+SC_BEGIN_UNLOAD event is not propagated to the ruby handler.
 
 
 C++ object manipulation

plugins/ruby/ruby.cpp

@@ -181,7 +181,9 @@ DFhackCExport command_result plugin_onstatechange ( color_ostream &out, state_ch
         SCASE(MAP_UNLOADED);
         SCASE(VIEWSCREEN_CHANGED);
         SCASE(CORE_INITIALIZED);
-        SCASE(BEGIN_UNLOAD);
+	// if we go through plugin_eval at BEGIN_UNLOAD, it'll
+	// try to get the suspend lock and deadlock at df exit
+	case SC_BEGIN_UNLOAD : return CR_OK;
 #undef SCASE
     }