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>

cmd/containerd/main_windows.go

@@ -4,17 +4,18 @@ import (
 	"context"
 	"os"
 	"path/filepath"
-	"syscall"
 
 	"github.com/containerd/containerd/log"
 	"github.com/containerd/containerd/server"
+
+	"golang.org/x/sys/windows"
 )
 
 var (
 	defaultConfigPath = filepath.Join(os.Getenv("programfiles"), "containerd", "config.toml")
 	handledSignals    = []os.Signal{
-		syscall.SIGTERM,
-		syscall.SIGINT,
+		windows.SIGTERM,
+		windows.SIGINT,
 	}
 )