The new setting will allow to host the temporary backup files
into a specific target directory. Defaults to '$CFG->tempdir/backup'.
Calling make_backup_temp_directory() checks that the required sub-directory
will be properly created under the new target directory.
When we import courses or duplicate activities, the deduplicating nature of
the file_storage system means that we don't actually have to include the
real files in the backup - just their metadata from the file table.
The restoration process needs to know not to expect files from the backup
phase so a flag is set in moodle_backup metadata.
