unix: add Sysv shared memory support

Implements proposed API from https://golang.org/issue/46084. I chose
`SysvShmDesc` since it's a clearer name than `SysvShm`. Initially supports Darwin and Linux.

Solaris support has a blocker (https://golang.org/issue/46084#issuecomment-836980018)

For golang/go#46084

Change-Id: Ied0f768a74c448254adc3315348417825a7ec63e
GitHub-Last-Rev: befbd7af6b2f838753ac163ef387cebdb7957467
GitHub-Pull-Request: golang/sys#110
Reviewed-on: https://go-review.googlesource.com/c/sys/+/327830
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>

unix/example_sysvshm_test.go

@@ -0,0 +1,52 @@
+// Copyright 2021 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.
+
+//go:build (darwin && amd64) || (linux && !android)
+// +build darwin,amd64 linux,!android
+
+package unix_test
+
+import (
+	"log"
+
+	"golang.org/x/sys/unix"
+)
+
+func ExampleSysvShmGet() {
+
+	// create shared memory region of 1024 bytes
+	id, err := unix.SysvShmGet(unix.IPC_PRIVATE, 1024, unix.IPC_CREAT|unix.IPC_EXCL|0o600)
+	if err != nil {
+		log.Fatal("sysv shm create failed:", err)
+	}
+
+	// warning: sysv shared memory segments persist even after after a process
+	// is destroyed, so it's very important to explicitly delete it when you
+	// don't need it anymore.
+	defer func() {
+		_, err := unix.SysvShmCtl(id, unix.IPC_RMID, nil)
+		if err != nil {
+			log.Fatal(err)
+		}
+	}()
+
+	// to use a shared memory region you must attach to it
+	b, err := unix.SysvShmAttach(id, 0, 0)
+	if err != nil {
+		log.Fatal("sysv attach failed:", err)
+	}
+
+	// you should detach from the segment when finished with it. The byte
+	// slice is no longer valid after detaching
+	defer func() {
+		if err = unix.SysvShmDetach(b); err != nil {
+			log.Fatal("sysv detach failed:", err)
+		}
+	}()
+
+	// Changes to the contents of the byte slice are reflected in other
+	// mappings of the shared memory identifer in this and other processes
+	b[42] = 'h'
+	b[43] = 'i'
+}