This enables the handlers to be used for hosts which use a non-standard
port, such as is offered by some commercial services. Default to port
22, so it's optional and backwards-compatible.
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".
Archive names must be unique, so if the backup runs more than once per
day with the previous default, it will throw and error because of the
duplicate archive name. This is also consistent with the addition of
the keephourly prune option.
