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_openbsd.go

@@ -1,14 +1,6 @@
 package fs
 
-import (
-	"syscall"
-
-	"github.com/restic/restic/internal/restic"
-)
-
-func nodeRestoreSymlinkTimestamps(_ string, _ [2]syscall.Timespec) error {
-	return nil
-}
+import "github.com/restic/restic/internal/restic"
 
 // nodeRestoreExtendedAttributes is a no-op on openbsd.
 func nodeRestoreExtendedAttributes(_ *restic.Node, _ string) error {