reopen file.
	(cache_btell, cache_bseek, cache_bflush, cache_bstat): Return -1 on
	bfd_cache_lookup failure.
	(cache_bread, cache_bwrite): Return 0 on the same.
	* bfdwin.c (bfd_get_file_window): Likewise.
	* hppabsd-core.c (hppabsd_core_core_file_p): Likewise.
	* sco5-core.c (sco5_core_file_p): Likewise.
	* trad-core.c (trad_unix_core_file_p): Likewise.