[racket] sync and input ports

From: Laurent (laurent.orseau at gmail.com)
Date: Sat Nov 10 15:22:18 EST 2012

Yes, that's what I did.
The code is here:

The commented main is the single-sync version, which does not work as
intended (i.e., clients can get blocked when several clients are
connected), and the uncommented main is with threads, which does work.

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