Download Slackware package files

Provide information about dependencies that have been packaged, but are uninstalled

Provide for arm systems and repositories

Fix error when initial download from repos was interrupted (so no packages_data file) by checking for the latter
# I think this is OK now

As part of dealing better with SLackware packages, do not assume that if there is a Slack package, there isn't a Salix one
#this seems to be happening at the moment; e.g. itstool
