dvd-sector-copy.c: Show overall progress if we're simply continuing.

dvd-sector-copy.c: Show overall progress if we're simply continuing.

If we're supposed to write a single region extending to the end of the
disc then show the percentage completion in terms of the absolute
position in the disc rather than as a percentage of the number of
sectors we're supposed to copy.  This makes the percentage shown easier
to relate to that displayed by a previous interrupted operation.