replace sys Sequential funcs with moby/sys/sequential

These functions were originally copied from the docker / moby repository in
4a7a8efc2d. Migrating these functions to use the
github.com/moby/sys/sequential module allows them being shared between moby,
docker/cli, and containerd, and to allow using them without importing all of sys
which also depends on hcsshim and more.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

vendor/github.com/moby/sys/sequential/doc.go

@@ -0,0 +1,15 @@
+// Package sequential provides a set of functions for managing sequential
+// files on Windows.
+//
+// The origin of these functions are the golang OS and windows packages,
+// slightly modified to only cope with files, not directories due to the
+// specific use case.
+//
+// The alteration is to allow a file on Windows to be opened with
+// FILE_FLAG_SEQUENTIAL_SCAN (particular for docker load), to avoid eating
+// the standby list, particularly when accessing large files such as layer.tar.
+//
+// For non-Windows platforms, the package provides wrappers for the equivalents
+// in the os packages. They are passthrough on Unix platforms, and only relevant
+// on Windows.
+package sequential