dump: Disallow \n in message IDs

When we switch to using regular Xapian queries in the dump format, \n
will cause problems, so we disallow it.  Specially, while Xapian can
quote and parse queries containing \n without difficultly, quoted
queries containing \n still span multiple lines, which breaks the
line-orientedness of the dump format.  Strictly speaking, we could
still round-trip these, but it would significantly complicate restore
as well as scripts that deal with tag dumps.  This complexity would
come at absolutely no benefit: because of the RFC 2822 unfolding
rules, no amount of standards negligence can produce a message with a
message ID containing a line break (not even Outlook can do it!).

Hence, we simply disallow it.