net: add dial, listenStream and listenDatagram methods to netFD

This CL refactors the existing listenerSockaddr function into several
methods on netFD.

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

Update issue 5199

R=golang-dev, dave, alex.brainman, dvyukov, remyoudompheng
CC=golang-dev
https://codereview.appspot.com/12023043