Add backup hook for permission and ownership fixes

Backups can fail due to perm/ownership issues, since Aegir might not be able to read files created by the site (that are owned by www-data:www-data with mode 600, for example). Actually, I guess this'd also affect clones and migrations. At present, a verify task should fix perms/ownership, and then allow the backup task to succeed. However, it should be pretty easy to trigger those scripts during those tasks automatically.