PR 973

	* cache.c (enum cache_flag): New.
	(close_one): Save file mtime.
	(bfd_cache_lookup): Add flag arg, adjust all users.
	(bfd_cache_lookup_worker): Likewise.
	(cache_btell): Use CACHE_NO_OPEN and return abfd->where if file
	not open.
	(cache_bflush): Similarly, and return success of file not open.
	(cache_bseek): Use CACHE_NO_SEEK if SEEK_SET or SEEK_END.
	(cache_bstat): Use CACHE_NO_SEEK_ERROR.