unix: use unsafe.Slice instead of unsafeheader package

Go 1.18 is the minimum supported Go version and unsafe.Slice was
introduced in Go 1.17.

Change-Id: Ie5f1fad01f219e7b7a190de2c49676f366ad3bc7
Reviewed-on: https://go-review.googlesource.com/c/sys/+/428515
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Benny Siegert <bsiegert@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>

unix/sysvshm_unix.go

@@ -7,11 +7,7 @@
 
 package unix
 
-import (
-	"unsafe"
-
-	"golang.org/x/sys/internal/unsafeheader"
-)
+import "unsafe"
 
 // SysvShmAttach attaches the Sysv shared memory segment associated with the
 // shared memory identifier id.
@@ -34,12 +30,7 @@ func SysvShmAttach(id int, addr uintptr, flag int) ([]byte, error) {
 	}
 
 	// Use unsafe to convert addr into a []byte.
-	// TODO: convert to unsafe.Slice once we can assume Go 1.17
-	var b []byte
-	hdr := (*unsafeheader.Slice)(unsafe.Pointer(&b))
-	hdr.Data = unsafe.Pointer(addr)
-	hdr.Cap = int(info.Segsz)
-	hdr.Len = int(info.Segsz)
+	b := unsafe.Slice((*byte)(unsafe.Pointer(addr)), int(info.Segsz))
 	return b, nil
 }