bug#43028: 28.0.50; Add dired commands to navigate symbolic links

[Drew Adams]

> Yeah, they're not much used.  tabulated-list-mode uses them to
> widen/narrow columns, for instance, and...   that's all the special-mode
> usage of those keys I can remember.  :-)

Dired uses `M-{' and `M-}' to move among marked files.

Personally, I'd say leave navigation-to-symlinks commands
unbound to keys.  If such navigation becomes popular,
users will bind the commands to keys, and if users ask
for Emacs to bind them by default we can then ask which
keys to use, etc.