Refactored bind/connect from sock.go into netFD.connect(), as
a consequence newFD() doesn't accept laddr/raddr anymore, and
expects an (optional) call to netFD.connect() followed by a
call to netFD.setAddr().
Windows code is updated, but still uses blocking connect,
since otherwise it needs support for ConnectEx syscall.

R=brainman, rsc
CC=golang-dev
https://golang.org/cl/4303060