[plt-scheme] Servlet2 and standalone.ss

I've just checked in a new wizard to DrScheme to SVN (new pre-built
versions will be available tomorrow, presumably). To show the wizard
even if you are not running a new version of drscheme, set the

  PLTDRWIZARD

environment variable.

While this change certainly won't help people who are not paying
attention, hopefully at least those that pay attention have a hope now,
which seems better than the current state.

Robby