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/CPlugin.h

@@ -61,6 +61,7 @@ public:
 		~CPlugin();
 		
 		bool m_Debug;
+		cell* m_pNullStringOfs;
 	public:
 		inline const char* getName() { return name.c_str();}
 		inline const char* getVersion() { return version.c_str();}
@@ -90,6 +91,7 @@ public:
 		
 		const char* getStatus() const;
 		inline bool isDebug() const { return m_Debug; }
+		inline cell* getNullStringOfs() const { return m_pNullStringOfs; }
 	}; 
 	
 private: