improve backup mode

Up to now, we did initially a full backup and then just incremental
backups. But in the longer view, this is not a) not very efficient
and b) will cost a lot of storage and c) keep certain userdata
forever in the 2ndsitebackup.

Also we did not cleanup any orphaned files etc. so the cache
directory filled very quickly.

So we switch now to the following mode:

1. do an initial backup
2. do incremental backups afterwards for 60 days
3. every 60 days do a full backup.
4. remove everything older than the second last full backup, so
   older than 120 days.
5. cleanup the various files in the beginning.

This should give us a more stable and efficient 2ndsitebackup +
save some diskspace. On the other hand it requires more bandwidth.
stop the backup manually after a certain hour of the day, this  will properly finish the current backup, till that hour is reached
also print timestamp of the current subtarget
add archive_dir option
introduce lockfile handling so we don't have two backup processes battling with each another
store the backups longer
introduce soft_failing_targets

The idea is we have a couple of targets that will cause problems over
and over again. Maybe they are currently not backupable or whatever.
To not always put our hands on if such a target is running, we can
add them to a list of softfailing targets, which won't cause
2ndsitebackup to abort immediately, as we would do otherwise in case
of a failure.
manage soft_failing_target file
fix stupid typo
new style for 2.7