Fixing mkdir permissions (using $CFG->directorypermissions)

Hi there! First of all, thank you very much for this plugin! :)

We at the Federal University of Santa Catarina (Brazil) were facing some issues while running our backup script on the "vpl_data" folder. This was due to the hardcoded 0700 permission. I fixed that by using the "$CFG->directorypermissions" from Moodle's config file.

Thanks!