1 order of opening database files
6 - try to read an options file .opt or .OPT
7 if found, merge options with those passed,
8 and use the successful case for extensions
9 - else, if the last path component contains an uppercase ascii
10 and no lowercase ascii, use uppercase extensions
11 - Options may select to open only index or only data.
12 - For both index and data, either traditional or new files are used
13 during normal operation, with new files preferred.
14 However, both sets may be open during copying.
15 - if no read/write mode is explicitly selected,
16 readonly is used if a TXT or MST file exists and is readonly.
17 (By index file permissions in index-only mode).
22 - if TXT exists, use it, done
23 - if traditional MST/XRF exist,
24 and they are writable or writing was not explicitly selected,
26 - create TXT and copy any available readonly data
28 If for a TXT no valid PTR exists, create one.
33 - if OXI exits, use it, done
34 - if traditional index files exist, open readonly
35 - else, OR if writing is wanted
36 create OXI for writing, copy old index