Fix Windows symlink issues

On Windows, symlinks require admin priveleges.  For directories,
it is possible to create junction points instead.  This PR

* Provide a message on Windows if symlink creation fails, indicating
  that symlinks require admin priveleges
* Automatically attempts to create junction points whenever symlinking
  a directory
Merge pull request #7274 from JuliaLang/kms/win_symlink_fix

RFC: Fix Windows symlink issues