[plt-scheme] install script comments

I just downloaded and installed DrScheme 207 for Solaris.  I think the install 
script is great, however I think a couple improvements could be made.

When creating symbolic links, if the file exists already as a file (not just a 
link), then installation fails.  It would be nice if prompted to delete the 
file or skip creating a link.

The next time I run the install script I'm prompted to delete the plt 
directory I just created.  An option to use the directory and skip the 
extraction step would've saved a ton of time.

Thanks,
David