Remove padding on the message-flags.

  This was only used for the $FLAG expansion in the
 index_format lua-string.  Do the padding there instead.
  Document our data-extraction from the text/plain part.

  Partly because I suspect the code is wrong, even though it
 does indeed work.  Partly for the benefit of other developers.
  Added/updated comments for  CMessage::add_attachments_to_mail()

  Because it is both new and complex.