return available (i.e. released!) release versions
It's not perfect that this needs manual updating. On the other hand, this provides control over what versions are (trivially) visible to users.
MR includes tidy up of the main() function: it is now a little bit more explicit which action requires a release, and also which action has a logfile (I hope).
This is a basis on which we can extend the status
command (to list installed releases if no release is specified).