Rxnpatch: Support multiple revisions of clusters.

Prepares for fixing [bug 7225].
Rxnpatch: Support creation of multiple clusters on the same day.

If a cluster already exists, a revision number will be appended
to the timestamp (starting with "-02") to make it unique.

Fixes [bug 7225].
Rxnpatch: Added some cluster creation paranoia.

Cluster creation now aborts (instead of making an invalid tar file)
if a contained filename is too long (ie 100 characters or more).
This is unlikely to ever happen, but...