prune {validate, mark}: preserve input parameter

prune {validate, mark}: preserve input parameter

While the current way to detect settings like { 'keep-last' => 0 } is
concise, it's also wrong, because but the delete operation is visible
to the caller. This resulted in e.g.
    # $hash is { 'keep-all' => 1 }
    my $s = print_property_string($hash, 'prune-backups');
    # $hash is now {}, $s is 'keep-all=1'
because validation is called in print_property_string. The same issue
is present when calling prune_mark_backup_group.

Because validation complains when keep-all and something else is set,
this shouldn't have caused any real issues, besides vzdump with
keep-all=1 wrongly taking the removal path, but without any settings,
so not removing anything:
    INFO: prune older backups with retention:
    INFO: pruned 0 backup(s)

Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>