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).

imapserver/server.go

@@ -2731,7 +2731,7 @@ func (c *conn) cmdAppend(tag, cmd string, p *parser) {
 				Received:      tm,
 				Flags:         storeFlags,
 				Keywords:      keywords,
-				Size:          size,
+				Size:          size + int64(len(msgPrefix)),
 				MsgPrefix:     msgPrefix,
 			}