net: separate pollster initialization from network file descriptor allocation

Unlike the existing net package own pollster, runtime-integrated
network pollster on BSD variants, actually kqueue, requires a socket
that has beed passed to syscall.Listen previously for a stream
listener.

This CL separates pollDesc.Init (actually runtime_pollOpen) from newFD
to allow control of each state of sockets and adds init method to netFD
instead. Upcoming CLs will rearrange the call order of runtime-integrated
pollster and syscall functions like the following;

- For dialers that open active connections, runtime_pollOpen will be
  called in between syscall.Bind and syscall.Connect.

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

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

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

Update #5199

R=dvyukov, alex.brainman, minux.ma
CC=golang-dev
https://golang.org/cl/8608044

src/pkg/net/file_unix.go

@@ -67,6 +67,10 @@ func newFileFD(f *os.File) (*netFD, error) {
 		closesocket(fd)
 		return nil, err
 	}
+	if err := netfd.init(); err != nil {
+		netfd.Close()
+		return nil, err
+	}
 	netfd.setAddr(laddr, raddr)
 	return netfd, nil
 }