Existing implementation does not provide a way to set options such as
SO_REUSEPORT, that has to be set prior the socket being bound.

New exposed API:
pkg net, method (*ListenConfig) Listen(context.Context, string, string) (Listener, error)
pkg net, method (*ListenConfig) ListenPacket(context.Context, string, string) (PacketConn, error)
pkg net, type ListenConfig struct
pkg net, type ListenConfig struct, Control func(string, string, syscall.RawConn) error
pkg net, type Dialer struct, Control func(string, string, syscall.RawConn) error

Fixes #9661

Change-Id: If4d275711f823df72d3ac5cc3858651a6a57cccb
Reviewed-on: https://go-review.googlesource.com/72810
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>