planet2: installation-wide config of default `raco pkg' scope

With either
  configure --enable-pkgscope=installation
or
  raco pkg config -i --set default-scope installation
the default scope of `raco pkg' actions can be changed from
user-specific to installation-wide.

We considered trying to guess when someone building Racket would prefer
installation-wide package scope by default. In particular, someone
building from source for in-place use seems likely to want
installation-wide scope by default. Then again, we don't want to
discourage in-place builds for Unix installations that are intended
for multiple users. So, no guessing for now.

Also, add a `--scope' argument to `raco pkg' commands, which is more in
line with other options, but keep `-i', etc., as shorthands.