Introduce --no-resume.

If the dss daemon (or the rsync process) is killed while a snaphot
is being created, e.g. because of a server shutdown, the latest
snapshot remains incomplete until it is removed by the usual shapshot
pruning mechanism.

This patch changes the snapshot creation behaviour if the
most recently created snapshot happens to be incomplete and the
new --no-resume option is not given. In this case the directory
of the incomplete snapshot is reused as the destination directory
for the the new snapshot.

This change saves disk space and reduces the snapshot creation time,
depending of course on how far the previous rsync process got before
it was interrupted.