C-d into nndoc and F yield bad GCC

[Simon Josefsson <jas@extundo.com>]

If I C-d into a multipart message, e.g., to read a multipart message
with the encrypted parts, and then press F on a part, the GCC header
holds the nndoc group name.  I'm using gcc-self.  This doesn't work,
the GCC simply fails saying nndoc isn't writable, which is true of
course, and then I lose the GCC.  I can save it to nndraft and then
move that article into my archive group, but the MML isn't translated
into MIME then.  If someone wants to look into it, I think there are
several problems:

* Gcc-self in a C-d nndoc group should gcc the original group.

* When Gcc fails, there should be some way to get to the MML
  translated message.  Perhaps save the Gcc prepared message into
  nndraft and inform the user?

(* I wouldn't have discovered this problem if the gnus-uu code were
   invoked on each textual MIME part, thus rendering inlined PGP
   parts.  But this is a known problem.)