beinstall: try to save progress from pkg updates.

This is primarily aimed at failed updates due to package conflicts, and
affects treatment of failed updates. Whereas before potentially a large
number of packages would need to be synced for each attempt, they can now
be persisted. Requires rsync. There may be better ways to implement this,
e.g. using secondary cache path that is only used on followup attempts and
then wiped on success, which avoids polluting current cache.