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>

fs/copy_unix.go

@@ -10,6 +10,7 @@ import (
 	"github.com/containerd/containerd/sys"
 	"github.com/containerd/continuity/sysx"
 	"github.com/pkg/errors"
+	"golang.org/x/sys/unix"
 )
 
 func copyFileInfo(fi os.FileInfo, name string) error {
@@ -63,5 +64,5 @@ func copyDevice(dst string, fi os.FileInfo) error {
 	if !ok {
 		return errors.New("unsupported stat type")
 	}
-	return syscall.Mknod(dst, uint32(fi.Mode()), int(st.Rdev))
+	return unix.Mknod(dst, uint32(fi.Mode()), int(st.Rdev))
 }