syscall: add Conn and RawConn interfaces

This change adds Conn and RawConn interfaces which can be used to
manipulate raw network connection end points typically represented as
socket descriptors.

Fixes #19435.

Change-Id: Ide2d28eeab91bfd27473ab47a87bec69950b64c9
Reviewed-on: https://go-review.googlesource.com/37913
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/syscall/net.go

+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package syscall
+
+// A RawConn is a raw network connection.
+type RawConn interface {
+	// Control invokes f on the underlying connection's file
+	// descriptor or handle.
+	// The file descriptor fd is guaranteed to remain valid while
+	// f executes but not after f returns.
+	Control(f func(fd uintptr)) error
+
+	// Read invokes f on the underlying connection's file
+	// descriptor or handle; f is expected to try to read from the
+	// file descriptor.
+	// If f returns true, Read returns. Otherwise Read blocks
+	// waiting for the connection to be ready for reading and
+	// tries again repeatedly.
+	// The file descriptor is guaranteed to remain valid while f
+	// executes but not after f returns.
+	Read(f func(fd uintptr) (done bool)) error
+
+	// Write is like Read but for writing.
+	Write(f func(fd uintptr) (done bool)) error
+}
+
+// Conn is implemented by some types in the net package to provide
+// access to the underlying file descriptor or handle.
+type Conn interface {
+	// SyscallConn returns a raw network connection.
+	SyscallConn() (RawConn, error)
+}