net: If we stop polling, remove any pending events for the socket
net: If we stop polling, remove any pending events for the socket

Fixes issue 1872.

R=rsc
CC=golang-dev, lars.pensjo
http://codereview.appspot.com/4559046

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang