Add a new `extra-dispatcher-file' configuration option.

This allows handling other requests implement additional course-related
functionality by the already running server.  The documentation needs
more work though: it refers to `dispatcher/c' which needs to be added,
and there is no real sample code for it.  (My use is too specific to be
used.)
MY PATCH!

To be rebased on newer changes from upstream.

(Also includes a pre-addition copy of main.rkt, kept just in case.)