RHEL7: fix gdb auto-load issues

- turn auto-load on - because we need to have the pretty printer on
- add /var/cache/abrt-di to auto-load safe-path
- add /var/cache/abrt-di to auto-load scripts-directory
- see https://bugzilla.redhat.com/show_bug.cgi?id=1163339 for more details

This pull request back ports #877 to the rhel7 branch.