[borg] replace "keep*" options with simpler "keep"

The effect of "keep*" options is not straightforward to understand, so
replacing it with a simpler "keep" option, which replicates the
functionality of other backupninja handlers. This also simplifies the
helper, as the use is then only asked how many days of backups to keep.

At the same time, we add "prune_options" which allows for the use of the
"keep*" options as well as other useful prune options, like "--prefix".