Support multi-extent iso files.

	Support multi-extent iso files.

	* grub-core/fs/iso9660.c (grub_iso9660_data): Remove first_sector.
	Add node.
	(grub_fshelp_node): Revamp. All users updated.
	(FLAG_*): New enum.
	(read_node): New function.
	(grub_iso9660_susp_iterate): Use read_node. Receive a node as argument.
	All users updated.
	(grub_iso9660_mount): Don't attempt to read sua when there is none.
	(get_node_size): New function.
	(grub_iso9660_iterate_dir): Use read_node. Agglomerate multi-extent
	entries.
	Fix memory leak on . and ..
	(grub_iso9660_read): Use read_node.
	(grub_iso9660_close): Free node.