Reject SMTP session from any client that sends illegally-long lines

Log a warning message and send a "500 Line too long" before disconnecting any SMTP client that sends lines > 998 characters in length. Technically, we can handle lines up to 1023 characters, but then we could get out of sync with the client if it sends exactly 1023 chars and then a new-line char (which we would interpret as a blank line, separating the message header and body) - so just punt the client who doesn't obey the rules of the protocol.