unicorn 4.8.1
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.