Should "su" change the USER environment variable?

[Steven Bellovin]

There's a philosophical question that has to be resolved first:  is the
purpose of 'su' to let you "become" the other user, or is it to extend
your permission while retaining your identity.  If you're trying to become
someone else, then USER should be changed; if not, then it should be left
alone.  I tend to subscribe to the second view.  Back in the mists of
time, when I was modifying ucbmail to work with MMDF, I used USER to
generate the 'From' line, but the real uid to generate a 'Sender' line.
That seemed to work out best, under the circumstances.