Introduce containerd-shim-runhcs-v1 on Windows

Implements the containerd-shim-runhcs-v1 shim on Windows for the runtime
v2 shim API.

Signed-off-by: Justin Terry (VM) <juterry@microsoft.com>

vendor/github.com/Microsoft/hcsshim/internal/longpath/longpath.go

@@ -0,0 +1,24 @@
+package longpath
+
+import (
+	"path/filepath"
+	"strings"
+)
+
+// LongAbs makes a path absolute and returns it in NT long path form.
+func LongAbs(path string) (string, error) {
+	if strings.HasPrefix(path, `\\?\`) || strings.HasPrefix(path, `\\.\`) {
+		return path, nil
+	}
+	if !filepath.IsAbs(path) {
+		absPath, err := filepath.Abs(path)
+		if err != nil {
+			return "", err
+		}
+		path = absPath
+	}
+	if strings.HasPrefix(path, `\\`) {
+		return `\\?\UNC\` + path[2:], nil
+	}
+	return `\\?\` + path, nil
+}