of the first message in the thread, or 0 if unknown (e.g. legacy message). When
a new message is created, the thread_id value of the message being replied-to
will be used, else if the new msg is not a reply, then the new message's number
will be the value used for the thread_id.
This "thread-id" feature (Deuce's idea) allows threads with a common parent to
remain logically grouped after the original messages in the thread have been
purged (though the original hiearchy will start to be lost).
smb_addmsg() was retrieving the *header* of the replied-to message (via FTN
REPLYID): optimized by just retrieving the index.
Moved unused SMB fixed header fields: times_downloaded and last_downloaded
to provide more logitech binary alignment of the fixed header fields. Since
these fields are not yet used, this should have no affect on existing bases.