set the port to listen on and really ensure that the
window of the fork()ed instance is closed.

works somewhat: it's now actually possible to listen for an
incoming connection and to close it again, but the second
connection attempt fails with 'XIO:  fatal IO error 11
(Resource temporarily unavailable)'. this could relate to the
fact that SDL uses threads internally and we're fork()ing
here...
Signed-off-by: Christian Beier <dontmind@freeshell.org>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>