mime-node: split out _mime_node_set_up_part

This is a code reorganization that should have no functional effect,
but will make future changes simpler, because a future commit will
reuse the _mime_node_set_up_part functionality without touching
_mime_node_create.

In the course of splitting out this function, I noticed a comment in
the codebase that referred to an older name of _mime_node_create
(message_part_create), where this functionality originally resided.
I've fixed that comment to refer to the new function instead.

Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>