net: rearrange the call order of runtime-integrated network pollster and syscall functions

This CL rearranges the call order for raw networking primitives like
the following;

- For dialers that open active connections, pollDesc.Init will be
  called before syscall.Connect.

- For stream listeners that open passive stream connections,
  pollDesc.Init will be called just after syscall.Listen.

- For datagram listeners that open datagram connections,
  pollDesc.Init will be called just after syscall.Bind.

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

Update issue 5199

R=golang-dev, alex.brainman
CC=golang-dev
https://codereview.appspot.com/12730043