Clarify the method of WRITE-MIME-MESSAGE-BODY-PART specialized on

Clarify the method of WRITE-MIME-MESSAGE-BODY-PART specialized on
<MESSAGE> instances, and fix two bugs in it:

1. Message part selectors may be empty lists, in which case the whole
   message body is meant; this may arise, for instance, when a MIME
   message is sent with a wholly unrecognize Content-Type (like
   application/pkcs7-mime), and the user tries to save the MIME entity
   that represents the whole message to a file.

2. WRITE-HEADER-FIELDS takes a list of header field strings, not a
   message.  How this never arose, I don't know.