Add Windows node support.

The syscall.Stat_t doesn't exist on Windows, so it is replaced by an interface,
which Windows can fill out, and field access is replaced by function calls.

Common Unix functionality is put into "node_unix.go", so there is less boilerplate.

Symlinks are skipped on Windows, since they require admin privileges.

node_windows.go

@@ -0,0 +1,66 @@
+package restic
+
+import (
+	"os"
+	"syscall"
+)
+
+func (node *Node) OpenForReading() (*os.File, error) {
+	file, err := os.OpenFile(node.path, os.O_RDONLY, 0)
+	if os.IsPermission(err) {
+		return os.OpenFile(node.path, os.O_RDONLY, 0)
+	}
+	return file, err
+}
+
+// mknod() creates a filesystem node (file, device
+// special file, or named pipe) named pathname, with attributes
+// specified by mode and dev.
+var mknod = func(path string, mode uint32, dev int) (err error) {
+	panic("mknod not implemented")
+}
+
+// Windows doesn't need lchown
+var lchown = func(path string, uid int, gid int) (err error) {
+	return nil
+}
+
+func (node Node) restoreSymlinkTimestamps(path string, utimes [2]syscall.Timespec) error {
+	return nil
+}
+
+type statWin syscall.Win32FileAttributeData
+
+func toStatT(i interface{}) (statT, bool) {
+	if i == nil {
+		return nil, false
+	}
+	s, ok := i.(*syscall.Win32FileAttributeData)
+	if ok && s != nil {
+		return statWin(*s), true
+	}
+	return nil, false
+}
+
+func (s statWin) dev() uint64   { return 0 }
+func (s statWin) ino() uint64   { return 0 }
+func (s statWin) nlink() uint64 { return 0 }
+func (s statWin) uid() uint32   { return 0 }
+func (s statWin) gid() uint32   { return 0 }
+func (s statWin) rdev() uint64  { return 0 }
+
+func (s statWin) size() int64 {
+	return int64(s.FileSizeLow) | (int64(s.FileSizeHigh) << 32)
+}
+
+func (s statWin) atim() syscall.Timespec {
+	return syscall.NsecToTimespec(s.LastAccessTime.Nanoseconds())
+}
+
+func (s statWin) mtim() syscall.Timespec {
+	return syscall.NsecToTimespec(s.LastWriteTime.Nanoseconds())
+}
+
+func (s statWin) ctim() syscall.Timespec {
+	return syscall.NsecToTimespec(s.CreationTime.Nanoseconds())
+}