be-handle-mail now handles non-text comments.

be-handle-mail now handles non-text comments.

This required replacing both the codec-wrapped sys.stdin _and_ the raw
sys.__stdin__ with StringIO(stdin).  becommands/comment will use only
one or the other depending on the comment's content type.

Caveat: Get_body_type only grabs the body and type of the first
non-mulitpart section, which may not be what the user expects.

Todo: Add multiple comments for each part of a multipart message, like
we do in interfaces/xml/be-mbox-to-xml.