reply: Move reply citation printing to the recursive MIME walk

reply: Move reply citation printing to the recursive MIME walk

This makes more logical sense, since it makes the recursive printer
responsible for the entire reply body and lets it start at the root of
the MIME tree instead of the first child.  (We could move reply header
creation in there, too, but if we ever support proper reply to
multiple messages, we'll want just one set of reply headers computed
from the entire message set and many bodies.)