core: twistd blocks when running in daemon mode

twistd is only able to run the core in foreground mode (ie: -n). when daemoninzed, the process is stuck at nearly 100% CPU usage, and ltrace shows that zmq is raising the "Interrupted System Call" error continously.

(from redmine: created on 2016-02-18, closed on 2016-02-23, relates #7137 (closed), relates #7536 (closed))