order of opening database files


*	checking options

-	try to read an options file .opt or .OPT
	if found, merge options with those passed,
	and use the successful case for extensions
-	else, if the last path component contains an uppercase ascii
	and no lowercase ascii, use uppercase extensions
-	Options may select to open only index or only data.
-	For both index and data, either traditional or new files are used
	during normal operation, with new files preferred.
	However, both sets may be open during copying.
-	if no read/write mode is explicitly selected,
	readonly is used if a TXT or MST file exists and is readonly.
	(By index file permissions in index-only mode).


*	checking data

-	if TXT exists, use it, done
-	if traditional MST/XRF exist,
	and they are writable or writing was not explicitly selected,
	use them as is, done
-	create TXT and copy any available readonly data

If for a TXT no valid PTR exists, create one.


*	checking index

-	if OXI exits, use it, done
-	if traditional index files exist, open readonly
-	else, OR if writing is wanted
	create OXI for writing, copy old index