More progress:

Abstracted many fs funcs with helper class.
Removed much duplicated code.
Fixed ReadFile assuming FS errors for ValveFS.
Added ValveFS support for ReadFileString, WriteFile, WriteFileString, DeleteFile.
Added missing param in doc for OpenFile.

plugins/include/files.inc

@@ -136,17 +136,25 @@ native bool:ReadDirEntry(Handle:dir, String:buffer[], maxlength, &FileType:type=
  *
  * @param file			File to open.
  * @param mode			Open mode.
+ * @param use_valve_fs  If true, the Valve file system will be used instead.
+ *                      This can be used to find files existing in any of
+ *                      the GAME search paths, rather than solely files
+ *                      existing directly in the gamedir.
  * @return				A Handle to the file, INVALID_HANDLE on open error.
  */
-native Handle:OpenFile(const String:file[], const String:mode[]);
+native Handle:OpenFile(const String:file[], const String:mode[], bool:use_valve_fs=false);
 
 /**
  * Deletes a file.
  *
  * @param path			Path of the file to delete.
+ * @param use_valve_fs  If true, the Valve file system will be used instead.
+ *                      This can be used to find files existing in any of
+ *                      the GAME search paths, rather than solely files
+ *                      existing directly in the gamedir.
  * @return				True on success, false otherwise.
  */
-native bool:DeleteFile(const String:path[]);
+native bool:DeleteFile(const String:path[], bool:use_valve_fs=false);
 
 /**
  * Reads a line from a text file.