[plt-scheme] Web server examples

I haven't checked whether this is already fixed in SVN, but in the 
distribution version of 299.200 (which I installed from 
mz-299.200-bin-i386-linux.sh), a number of the web server examples are 
broken:

add.ss and quiz.ss need to have the require of "servlet-helpers.ss" removed.

dir.ss needs a call to path->string.

And the examples in the examples/compound seem to have some issues too.

Anton