Changed user' message pointer/last-read/scan-config storage to a new file

(data/user/*.subs - in familar .ini syntax)
indexed by the sub-board internal rather than the old flat binary file using
the troublesome "pointer index" (ptridx) as an offset. The old files
(data/user/ptrs/*.ixb) will still be read in when the new file doesn't exist,
for upgrading users automatically to the new method of pointer storage.
This should resolve long-standing issues around users' new-scan pointers and
configurations getting corrupted when the sysop inserts/remove sub-boards.
However, if you change a sub-board's internal code, that invalidates the users'
pointers and scan configurations - so that may need to be addrsesed via SCFG.
The subs' ptridx values are still be used for storing QWKnet hub pointers, for
now, but that too will soon be addressed with a new file format for those ptrs
too.