Sieve library

[Sergey Poznyakoff]

Hello,

Quoting TODO:
> - run as daemon, sieveing mail on arrival (need interface for notification
>   of message arrival, this is supported by imap, but we'll have to fake
>   for pop and local spools my polling, why can't you select() on a unix
>   file?)

I'd rather propose to make it a library, say libsieve. Then this library can
be used in writing an MDA which will sieve the mail immediately upon
arrival. Maybe we could make such delivery agent a part of the project, too.
How do you think?

By the way, during last two days I was experimenting using
guimb+sieve.scm as an MDA on my machine. It showed that the approach is
worth while, but using libsieve would be much faster and would consume
less memory. 

-Sergey