Currently, sa-update replaces the entire set of channel rules when it updates.
This could possible lead to a condition where an update destroys a set of
working rules in favor of a set that doesn't work as well. Adding an option to
archive the current set prior to updating would provide protection against this
in that the admin could then manually restore a previous rule set.

One idea for doing this would be to create an archive subdirectory and tgz the
current rules into a file named by channel version number. A limited number of
versions should be kept.

Both the archiving and the number of archived versions should be configurable
options-- maybe --noarchive and --numarchives=n.