emacs: save undisplayer function for MIME parts

emacs: save undisplayer function for MIME parts

For some kinds of MIME parts (at least images), our trickery with
overlays will not work, so save the more drastic function created by
Gnus that actually deletes the part from the buffer. In an ideal world
we would return this function as (part of) a value, but here the call
stack is too complicated for anything that simple, so we stash it in
the part plist and rely on that being preserved (unlike the mm handle,
which is transient).