add message size consistency check

the bulk of a message is stored on disk. a message prefix is stored in the
database (for prefixed headers like "Received:"). this adds a check to ensure
Size = prefix length + on-disk file size.

verifydata also checks for this now.

and one older and one new (since yesterday) bug was found. the first when
appending a message without a header/body section (uncommon). the second when
sending messages from webmail with localserve (uncommon).

queue/queue.go

@@ -215,7 +215,7 @@ func Add(ctx context.Context, log *mlog.Log, senderAccount string, mailFrom, rcp
 			err := acc.Close()
 			log.Check(err, "closing account")
 		}()
-		m := store.Message{Size: size}
+		m := store.Message{Size: size, MsgPrefix: msgPrefix}
 		conf, _ := acc.Conf()
 		dest := conf.Destinations[mailFrom.String()]
 		acc.WithWLock(func() {