* listing.c (file_info_struct): Remove FILE, add POS.

	(last_open_file_info, last_open_file): New; a one entry FILE* cache.
	(file_info): Don't open the file.
	(buffer_line): Check for the file in the last_open cache, updating
	as necessary.
	(print_source): Don't reference file_info->file.
	(listing_listing): Likewise.
	(listing_print): Close the file in the cache, if any.