Rename `mime-gnus-handle-*' to `mmgnus-*'.

Rename `mime-gnus-handle-*' to `mmgnus-*'.
(mm-handle-body): Rename from `mm-handle-buffer'.
(mm-handle-set-body): Rename from `mm-handle-set-buffer'.
(mm-make-handle): New argument `parent' and `header'.  If encoding is `nil',
do not convert to string.
(mm-dissect-buffer-header): Rename argument `parent' to `handle'.  Set raw
header string to header slot of `mmgnus-entity'.
(mm-dissect-buffer): Rename argument `parent' to `handle'.  When use
`MIME-View' mode as viewer, parse `message/*' part.
(mm-dissect-singlepart): Use `mm-handle-set-body' and `mm-handle-body'.
(mm-dissect-message): New function.
(mm-dissect-multipart): Rename argument `parent' to `handle'.
(mm-dissect-multipart): Set node ID for each parts.
(mm-destroy-part): Use `mm-handle-body' and `mm-handle-set-body'.
(mm-insert-part): Likewise.