net: disable wild use of SO_REUSEPORT on BSD variants
net: disable wild use of SO_REUSEPORT on BSD variants

Fixes issue 2830 (again).

R=rsc
CC=golang-dev
http://codereview.appspot.com/5651083

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