Switch from package syscall to golang.org/x/sys

The syscall package is locked down and the comment in [1] advises to
switch code to use the corresponding package from golang.org/x/sys. Do
so and replace usage of package syscall with package
golang.org/x/sys/{unix,windows} where applicable.

  [1] https://github.com/golang/go/blob/master/src/syscall/syscall.go#L21-L24

This will also allow to get updates and fixes for syscall wrappers
without having to use a new go version.

Errno, Signal and SysProcAttr aren't changed as they haven't been
implemented in x/sys/. Stat_t from syscall is used if standard library
packages (e.g. os) require it. syscall.ENOTSUP, syscall.SIGKILL and
syscall.SIGTERM are used for cross-platform files.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>

container_linux_test.go

@@ -4,12 +4,12 @@ package containerd
 
 import (
 	"context"
-	"syscall"
 	"testing"
 
 	"github.com/containerd/cgroups"
 	"github.com/containerd/containerd/linux/runcopts"
 	specs "github.com/opencontainers/runtime-spec/specs-go"
+	"golang.org/x/sys/unix"
 )
 
 func TestContainerUpdate(t *testing.T) {
@@ -93,7 +93,7 @@ func TestContainerUpdate(t *testing.T) {
 	if int64(stat.Memory.Usage.Limit) != limit {
 		t.Errorf("expected memory limit to be set to %d but received %d", limit, stat.Memory.Usage.Limit)
 	}
-	if err := task.Kill(ctx, syscall.SIGKILL); err != nil {
+	if err := task.Kill(ctx, unix.SIGKILL); err != nil {
 		t.Error(err)
 		return
 	}
@@ -168,7 +168,7 @@ func TestShimInCgroup(t *testing.T) {
 	if len(processes) == 0 {
 		t.Errorf("created cgroup should have atleast one process inside: %d", len(processes))
 	}
-	if err := task.Kill(ctx, syscall.SIGKILL); err != nil {
+	if err := task.Kill(ctx, unix.SIGKILL); err != nil {
 		t.Error(err)
 		return
 	}