setenv's broken case when value is nil

[Noah Friedman]

address@hidden changed env.el on 2007-08-29 to define setenv-internal and
for setenv always to keep the variable name in process-environment, even if
value is nil.  In other words, he removed the distinction between the empty
string and nil, and therefore you can't remove variables from the
environment using setenv anymore.

Since all kinds of environment variables have meaning when they are
defined, even if they are just set to the empty string, this is a screw.