diskd and unlink

Hi everyone,

I'd like to make diskd use the unlink daemon instead of handling unlinks itself
in an attempt to lessen the amount of work being sent to the queue, and thus
(hopefully) mitigate the amount of times the disk callback is being recursively
called.

If noone objects I'll make this change to 2.HEAD, 2.7 and 2.6; I'll leave 3
to someone who knows more about the code post-code-shuffling.

Thanks,

Adrian

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