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/syscall_darwin.go

@@ -433,6 +433,11 @@ func SysctlKinfoProcSlice(name string) ([]KinfoProc, error) {
 
 //sys	sendfile(infd int, outfd int, offset int64, len *int64, hdtr unsafe.Pointer, flags int) (err error)
 
+//sys	shmat(id int, addr uintptr, flag int) (ret uintptr, err error)
+//sys	shmctl(id int, cmd int, buf *SysvShmDesc) (result int, err error)
+//sys	shmdt(addr uintptr) (err error)
+//sys	shmget(key int, size int, flag int) (id int, err error)
+
 /*
  * Exposed directly
  */
@@ -590,10 +595,6 @@ func SysctlKinfoProcSlice(name string) ([]KinfoProc, error) {
 // Msgget
 // Msgsnd
 // Msgrcv
-// Shmat
-// Shmctl
-// Shmdt
-// Shmget
 // Shm_open
 // Shm_unlink
 // Sem_open