Bug 14564: Set permissions on configuration backups

Bug 14564: Set permissions on configuration backups

This patch sets the group and group read permissions on
configuration backups, so that they can be downloaded
via the web staff interface.

Test plan:
0. Apply the patch
1. cp debian/scripts/koha-dump /usr/sbin/koha-dump
2. koha-run-backups --days 2 --output /var/spool/koha
3. Note *.tar.gz backups in /var/spool/koha/kohadev
now have root:kohadev-koha ownership and 640 permissions
4. vi /etc/koha/sites/kohadev/koha-conf.xml
5. Set backup_db_via_tools and backup_conf_via_tools to 1
6. echo 'flush_all' | nc -q 1 memcached 1121
7. koha-plack --restart kohadev
8. Go to http://localhost:8081/cgi-bin/koha/tools/export.pl
9. Click on "Export configuration"
10. Choose a file
11. Click Download configuration
12. Note that *.tar.gz file downloaded correctly

Signed-off-by: David Cook <dcook@prosentient.com.au>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>