DEB_MULEUCS_UNICODE=auto ?

[Dan Jacobson <jidanni@jidanni.org>]

DEB_MULEUCS_UNICODE=off makes emacs startup many times faster, but
when once a month, someone sends me a utf-8 email, the mail looks
broken.  I wish there was a DEB_MULEUCS_UNICODE=auto that would
activate it the first time we encounter unicode this session.

Or, with DEB_MULEUCS_UNICODE=off, there should be some warning message
in the minibuffer saying why the characters look bad when we first
encounter a unicode file this session, and what one should do about it
-- even if the only choice is 'restart emacs with no
DEB_MULEUCS_UNICODE=off'.