New method of handling of discs by FileCore.

FileCore always used to handle discs by identifying them by a (Name, Id)
pair. The name was always publically available, but the Id was hidden,
updated by FileCore, so that two discs with the same name would not be
mistaken by FileCore one for the other. This way of regarding discs worked
fine most of the time, however it had one or two drawbacks. If, from the
desktop, you opened some viewers, took the disc to a different machine,
copied some files to it, took it back and tried to copy the files off it you
would get an ambiguous disc error. Clicking on the floppy icon would enable
you to read the disc but would lose your viewers. All in all, this situation
is a bit unsatisfactory.

FileCore will be changed to treate discs as follows:

i) When a disc is identified internally by a (name, Id) pair, for example
when reading the contents of a cached directory or writing to an open file,
then that exact disc must be inserted. This does have the disadvantage that
modifying a disc with open files on it on a different machine causes you to
be well stuffed when you try inserting it later - FileCore just won't
recognise it as the same disc - fair enough really, as anything could have
been done to the disc on the different machine making any information held
about that disc by FileCore redundant.

ii) When a disc is identified by name, for example when accessing a file
from the desktop, then the chosen disc will be (a) the currently inserted
disc with that name, or (b) the disc FileCore last saw with that name. If
there are several (a)s , or if no (a)s and several (b)s then an ambiguous
disc error will be given. If there isn't an (a) or a (b) then the disc will
be requested for insertion. If there isn't an (a), but there is a (b), then
there is a fair chance that the information is cached and so no insertion
will be requested, but, if information on the disc is needed, then the exact
disc must be inserted.
