No documented way to turn off inheritable hooks

[senji-rsbackup]

I have pre-backup-hook and post-backup-hook set in my global configuration. For a small number of hosts in 2.0 I have {pre-,post-}backup-hook with no argument to disable the hook; however this no-longer works in 6.0 and I can't see a documented way to do this.

For now I have used 'pre-backup-hook true' as a placeholder.