Forking the whole process from deep within a library call does
not really work at all with apps that use multiple threads, i.e. every
reasonably modern GUI app. So, provide a non-forking listen function so
that the caller can decide if to fork, start a thread, etc.

This implementation adds a timeout parameter to be able to call the
listen function multiple times so that it's possible to do sth. else
in between, e.g. abort listening.
Signed-off-by: Christian Beier <dontmind@freeshell.org>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>