Zero byte appends

[Bron Gondwana]

Can anyone think of any good reason we should allow zero byte
files to be appended via IMAP?  Just that I'm currently going
over the reconstruct code - and that's one of the few places
where it actually changes things.

Reconstruct will always unlink a zero byte file and remove the
record - but IMAP APPEND will actually create one.  We should
align the behaviour one way or the other!

Bron.