email package's Parser to handle the three common line endings.
Certain protocols such as IMAP define CRLF line endings and it doesn't
make sense for the client app to have to normalize the line endings
before handing it message off to the Parser.

_parsebody(): Be more flexible in the matching of line endings for
finding the MIME separators.  Accept any of \r, \n and \r\n.  Note
that we do /not/ change the line endings in the payloads, we just
accept any of those three around MIME boundaries.