net: reduce unnecessary syscall.Sockaddr conversions

This CL makes IPAddr, UDPAddr and TCPAddr implement sockaddr
interface, UnixAddr is already sockaddr interface compliant, and
reduces unnecessary conversions between net.Addr, net.sockaddr and
syscall.Sockaddr.

This is in preparation for runtime-integrated network pollster for BSD
variants.

Update issue 5199

R=golang-dev, dave, bradfitz
CC=golang-dev
https://codereview.appspot.com/12010043