For now a pair of socket options SOL_SOCKET and SO_BINDTODEVICE
is supported on Linux only. I'd like to demote BindToDevice API
to syscall level because it's Linux dependent one.

In the near future, probably we may have a bit more portable
API that using IPROTO_IP/IPV6 level socket options to specify,
identify an inbound, outbound IP interface on incoming, outgoing
UDP and raw IP packets.

R=cw, golang-dev
CC=golang-dev
https://golang.org/cl/5447071