New script to setup a local repo for offline updates

[stormi]

With this script, one can setup a local repository from an archive which contains the files of a repository, and use it to install updates or additional packages.

The same script is meant to be used to update the repository. In order not to use too much space on the disk, we accept that the operation makes the repository unusable for the duration of the local copy.

The archive must contain a top level directory named after the repo type. For the updates repository, the script will look for ^updates/$.

Though I haven't produced any archive to test it, this is supposed to be usable not only for release updates, but also for other repos for testing, such as testing or candidates, if the need arises later (I bet it will).