Add menu_gamemenu client cvar to enable/disable the in-game menu

Apparently some people might prefer the old behavior. Fixes #2732 (closed)

qcsrc/client/main.qc

@@ -526,7 +526,7 @@ float CSQC_InputEvent(int bInputType, float nPrimary, float nSecondary)
 			Scoreboard_UI_Enable(0);
 			return true;
 		}
-		if (autocvar_cl_gamemenu && !isdemo() && cvar("_menu_gamemenu_dialog_available"))
+		if (autocvar_menu_gamemenu && !isdemo() && cvar("_menu_gamemenu_dialog_available"))
 		{
 			localcmd("\nmenu_showgamemenudialog\n");
 			return true;