Replace the guts of OldFileBase to make it v3.19 compatible

This appears to be enough to make hatchit.js work again and file
listings in webv4 (not sure beyond that).

This file is now just an unnecessary shim and should go away when
the consumers (hatchit and webv4) no longer need it.

I did not "port" support for the file properties:
- base
- ext
- datoffset
- opencount
- misc
- altpath

They don't appear to be needed.