Autopost not working on multi-site installation

The Autopost functionality works by inserting some message identifiers into the TROOT_post table, and then validating the presence of this entry through a dotfile on the mail server.

This is designed to facilitate the reuse of the same list across multiple sites. However, it doesn't, because posts from different sites result in a "duplicate key" error for the id column. The table definition should be updated to key on the URL and/or another, non-conflicting value.