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.

plugins/include/amxconst.inc

@@ -39,7 +39,15 @@
  */
 #define MAX_PLAYERS 32 /* Maximum number of players AMX Mod X supports */
 
-public stock const MaxClients; /* Maximum number of players the server supports */
+/**
+ * Maximum number of players the server supports 
+ */
+public stock const MaxClients; 
+
+/**
+ * Pass this into certain functions to act as a C++ NULL 
+ */
+public stock const NULL_STRING[1];
 
 /**
  * The maximum buffer size required to store a clients name.