vzdump: add config options for new notification backend

vzdump: add config options for new notification backend

- Add new option 'notification-target'
  Allows to select to which endpoint/group notifications shall be sent
- Add new option 'notification-policy'
  Replacement for the now deprecated 'mailnotification' option. Mostly
  just a rename for consistency, but also adds the 'never' option.
- Mark 'mailnotification' as deprecated in favor of 'notification-policy'
- Clarify that 'mailto' is ignored if 'notification-target' is set

Signed-off-by: Lukas Wagner <l.wagner@proxmox.com>