disk space problems: cachedir/packagedir aren't configurable

[wgwoods]

cachedir and packagedir are hardcoded to /var/tmp/system-upgrade and /var/lib/system-upgrade; if the user doesn't have space on /var (but has plenty of space in, say, /home) they can't upgrade their system.

But: if we let the user decide where to put the data, we need to make sure that whatever filesystem the data is on gets mounted during upgrade-prep.

This will require some extra jiggery-pokery with mounts and such to allow e.g. USB keys to be used as cache/install source.

[wgwoods]

See https://bugzilla.redhat.com/show_bug.cgi?id=989263 for an example where someone tried to work around this with symlinks.