[plt-scheme] automarking again

Eli Barzilay wrote:

> It's mostly there -- the interface that util.ss provides requires some
> port, it should be possible to write some relatively easy code that
> uses it with plain files.

To quote Matthew, "These little changes always create lots of work, 
somehow." I will seriously look into the handin server for next fall's 
offering, but in the meantime I will try to fix the /bin/sh scripts the 
tutors are using, because there's an assignment due in a week, and if I 
can sketch something out, the tutors can finish the implementation. 
Using cat to wrap module boilerplate around student code is 
aesthetically unpleasing, but I can do that right now, without 
understanding eval and with-handlers. --PR