* fhandler_disk_file.cc (DIR_NUM_ENTRIES): New define determining

	* fhandler_disk_file.cc (DIR_NUM_ENTRIES): New define determining
	minimum number of dir entries which fit into the readdir cache.
	(DIR_BUF_SIZE): Define globally as size of readdir cache.
	(struct __DIR_cache): New structure used for readdir caching on NT.
	(d_dirname): Accessor for struct __DIR_cache, use throughout.
	(d_pos): Ditto.
	(d_cache): Ditto.
	(fhandler_disk_file::opendir): Allocate __d_dirname to contain readdir
	cache on NT.
	(fhandler_disk_file::readdir): Use buf as pointer into readdir cache.
	Implement readdir caching.