Menu protection features

- added a '-nocustommenu' command line options. This is for disabling user menus that intentionally or accidentally disable crucial menu features.
- do not allow replacement of the main options menu. This menu contains engine-critical settings and may change at any time when new features get added. For now it will be the only one that gets protected but this may soon be extended to some of its submenus as well.

src/menu/menudef.cpp

@@ -54,6 +54,7 @@
 #include "vm.h"
 #include "types.h"
 #include "gameconfigfile.h"
+#include "m_argv.h"
 
 
 
@@ -520,6 +521,12 @@ static bool ReplaceMenu(FScanner &sc, DMenuDescriptor *desc)
 	DMenuDescriptor **pOld = MenuDescriptors.CheckKey(desc->mMenuName);
 	if (pOld != nullptr && *pOld != nullptr) 
 	{
+		if (desc->mMenuName == NAME_Optionsmenu)
+		{
+			sc.ScriptMessage("Cannot replace the main options menu!");
+			return true;
+		}
+
 		if (!CheckCompatible(desc, *pOld))
 		{
 			sc.ScriptMessage("Tried to replace menu '%s' with a menu of different type", desc->mMenuName.GetChars());
@@ -980,6 +987,7 @@ void M_ParseMenuDefs()
 				sc.ScriptError("Unknown keyword '%s'", sc.String);
 			}
 		}
+		if (Args->CheckParm("-nocustommenu")) break;
 	}
 	DefaultListMenuClass = DefaultListMenuSettings->mClass;
 	DefaultListMenuSettings = nullptr;