Synch with Gnus trunk.

Note: Julien has already sent a form to FSF to sign a paper.
2010-05-07  Julien Danjou  <julien@danjou.info>
 * gnus-art.el (gnus-mime-pipe-part): Add optional argument `cmd'; pass
 it to mm-pipe-part.

 * mm-decode.el (mm-pipe-part): Add optional argument `cmd'; use it if
 it is given.