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_linux.go

@@ -2319,6 +2319,11 @@ type RemoteIovec struct {
 //sys	PidfdOpen(pid int, flags int) (fd int, err error) = SYS_PIDFD_OPEN
 //sys	PidfdGetfd(pidfd int, targetfd int, flags int) (fd int, err error) = SYS_PIDFD_GETFD
 
+//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)
+
 /*
  * Unimplemented
  */
@@ -2400,10 +2405,6 @@ type RemoteIovec struct {
 // SetRobustList
 // SetThreadArea
 // SetTidAddress
-// Shmat
-// Shmctl
-// Shmdt
-// Shmget
 // Sigaltstack
 // Swapoff
 // Swapon