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_unix_notlinux.go

@@ -0,0 +1,21 @@
+//go:build !linux && unix
+
+package fs
+
+import (
+	"syscall"
+
+	"github.com/restic/restic/internal/restic"
+)
+
+// utimesNano is like syscall.UtimesNano, except that it skips symlinks.
+func utimesNano(path string, atime, mtime int64, typ restic.NodeType) error {
+	if typ == restic.NodeTypeSymlink {
+		return nil
+	}
+
+	return syscall.UtimesNano(path, []syscall.Timespec{
+		syscall.NsecToTimespec(atime),
+		syscall.NsecToTimespec(mtime),
+	})
+}