Introduce --min-complete.

Currently dss cowardly refuses to remove the last complete snapshot
even if disk space is low, and fails if there is not enough disk space
left for a second snapshot. However, in some situations it is more
important to have a recent snapshot and to to keep dss up and running.

This commit introduces a new integer option, --min-complete, which
defaults to one to resemble the old behaviour.

If it is set to zero, dss will happily remove the last complete
snapshot, even if it is used as the reference directory for rsync's
--link-dest option. This is dangerous, but it's the only way to keep
dss going.

Conversely, --min-complete may be set to a value greater than one
to guarantee there is always a certain number of complete snapshots
available.