The default `raco pkg' mode should work right for a
multiple-version installation (because everything in
Racket should work in a multiple-version installation).
Along the same lines, `raco pkg' should work if the
installation directory is unwriteable. So, the default
mode is user-specific and version-specific.
Use `--shared' or `-s' for user-specific, all-version
installs.
By default, `raco pkg show' now shows packages installed
in all three modes (installation-wide, user- and version-
specific, and user-specific all-version). Use `-i', `-u',
or `-s' to show just one of them.
