[Dovecot] SIS Implementation

[Jason Fesler]

> Hard links would be the simplest implementation without needing a
> separate database. Sure you could implement that too if you wanted to.

It would be worth checking the limits for hard links, and making sure they 
are suitable for a large mail system using this scheme, without having a 
fallback plan of some sort.

Looks like UFS hardlink limit is 32767; ext2 32000; reiser and jfs, 65535.
http://www.dirvish.org/viewcvs/dirvish_1_2/FAQ.html?rev=2  see
"Could linking between images be limited by a maximum link count?"