This fixes a couple problems when exiting due to signal:

 * connection threads exit without closing client socket
   (so client blocks waiting for response that never comes)

 * listen_queue_thread blocks on poll() until a new connection comes in
   (because no poll timeout specified)

-- kevin brintnall

git-svn-id: svn://svn.oetiker.ch/rrdtool/trunk/program@1525 a5681a0c-68f1-0310-ab6d-d61299d08faa