argument to the constructor -- defaulting to true -- which is
different than Anthony's approach of using global state.

parse(), parsestr(): Grow a `headersonly' argument which stops parsing
once the header block has been seen, i.e. it does /not/ parse or even
read the body of the message.  This is used for parsing message/rfc822
type messages.

We need test cases for the non-strict parsing.  Anthony will supply
these.

_parsebody(): We can get rid of the isdigest end-of-line kludges,
although we still need to know if we're parsing a multipart/digest so
we can set the default type accordingly.