fs: Refactor UtimesNano replacements

Previously, nodeRestoreTimestamps would do something like

	if node.Type == restic.NodeTypeSymlink {
	    return nodeRestoreSymlinkTimestamps(...)
	}
	return syscall.UtimesNano(...)

where nodeRestoreSymlinkTimestamps was either a no-op or a
reimplementation of syscall.UtimesNano that handles symlinks, with some
repeated converting between timestamp types. The Linux implementation
was a bit clumsy, requiring three syscalls to set the timestamps.

In this new setup, there is a function utimesNano that has three
implementations:

* on Linux, it's a modified syscall.UtimesNano that uses
  AT_SYMLINK_NOFOLLOW and AT_FDCWD so it can handle any type in a single
  call;
* on other Unix platforms, it just calls the syscall function after
  skipping symlinks;
* on Windows, it's the modified UtimesNano that was previously called
  nodeRestoreSymlinkTimestamps, except with different arguments.

internal/fs/node_linux_test.go

@@ -0,0 +1,19 @@
+package fs
+
+import (
+	"io/fs"
+	"strings"
+	"testing"
+
+	"github.com/restic/restic/internal/errors"
+	"github.com/restic/restic/internal/restic"
+	rtest "github.com/restic/restic/internal/test"
+)
+
+func TestRestoreSymlinkTimestampsError(t *testing.T) {
+	d := t.TempDir()
+	node := restic.Node{Type: restic.NodeTypeSymlink}
+	err := nodeRestoreTimestamps(&node, d+"/nosuchfile")
+	rtest.Assert(t, errors.Is(err, fs.ErrNotExist), "want ErrNotExist, got %q", err)
+	rtest.Assert(t, strings.Contains(err.Error(), d), "filename not in %q", err)
+}