mhshow/mhstore: Removed support for retrieving message/external-body parts.

These tools won't download the contents automatically anymore. Instead,
they print the information needed to get the contents. If someone should
really receive one of those rare message/external-body messages, he can
do the job manually. We save nearly a thousand lines of code. That's worth
it!
(The profile entry `nmh-access-ftp' and sbr/ruserpass.c for reading
~/.netrc are gone now.)