dvd-sector-copy.c (main): Add a flag to track retrying.

dvd-sector-copy.c (main): Add a flag to track retrying.

Previously we'd try to determine whether retry was intended from whether
there are (begin/stop) events in the queue.  This is bad because we
might have tried to read an empty retry file, in which case we'd still
have no events, but the intend would clearly be to do nothing rather
than re-read the entire input disc.