VFS: Fix various things

- The "ALL" fake pathID is replaced by what does SM, having a public var NULL_STRING which will acts as NULL when needed.
  To make compiler accepting public array, this patch was needed: https://hg.alliedmods.net/sourcemod-central/rev/b12f329def09
- The offset thing in read_dir: considering that's something very specific to this native and that implementation in CDirectory doesn't make sense because of the offset compatibility for windows, all code is now in the native.

amxmodx/amxmodx.cpp

@@ -29,7 +29,7 @@ static cell AMX_NATIVE_CALL get_xvar_id(AMX *amx, cell *params)
 	char* sName = get_amxstring(amx, params[1], 0, len);
 	cell ptr;
 
-	if (!strcmp(sName, "MaxClients"))
+	if (!strcmp(sName, "MaxClients") || !strcmp(sName, "NULL_STRING"))
 	{
 		return -1;
 	}