#822 Enable re-retrieval of reclassfied messages

[C.M Afsal]

It would have been quite useful to have feature by
which an email client can retrieve a reclassified message
from popfile. This is quite useful when one changes the
classification of a message from a bucket for which
quarantine has been enabled.

I guess this change requires popfile t change from a
simple proxy to behave like a real server. So I
understand the popfile team may not be in favor of this.
If that's the case, how about providing an operation to
undo the quarantine operation?