fix races/error handling in worker SIGQUIT handler

fix races/error handling in worker SIGQUIT handler

This protects us from two problems:

1) we (or our app) somehow called IO#close on one of the sockets
   we listen on without removing it from the readers array.
   We'll ignore IOErrors from IO#close and assume we wanted to
   close it.

2) our SIGQUIT handler is interrupted by itself.  This can happen as
   a fake signal from the master could be handled and a real signal
   from an outside user is sent to us (e.g. from unicorn-worker-killer)
   or if a user uses the killall(1) command.