mox was already strict in its "\r\n.\r\n" handling for end-of-message in an
smtp transaction.
due to a mostly unrelated bug, sequences of "\nX\n", including "\n.\n" were
rejected with a "local processing error".
the sequence "\r\n.\n" dropped the dot, not necessarily a big problem, this is
unlikely to happen in a legimate transaction and the behaviour not
unreasonable.
we take this opportunity to reject all bare \r. we detect all slightly
incorrect combinations of "\r\n.\r\n" with an error mentioning smtp smuggling,
in part to appease the tools checking for it.
smtp errors are 500 "bad syntax", and mention smtp smuggling.
