bg worker: patch 3 of 6 - sockets

This patch adds the capability for the coordinator to listen on sockets 
while waiting for imessages to arrive. Before the coordinator just slept 
until a signal arrives, notifying the coordinator about an internal message.

Major caveat here: I'm using pselect(), which might still not be 
portable enough. The work-around for platforms on which a signal doesn't 
interrupt select has been removed as well. I can't think of any way to 
support platforms as broken as that.