Improve old reports deletion

Improve old reports deletion

Use the last modification time to sort the old reports directories. The
previously used ctime is not the creation time but the time at which the
directory meta data was last modified.

Creating the old report list don't use temporary files nor the external
sort command. Everything is kept and sorted in memory.

The code make sure the report directory looks like one of our report
directory. This is a safety to avoid deleting an unrelated directory.

Currently this safety mechanism only detects directories created with in
index_tree set to "file".