[Toybox] [landley/toybox] Add ln -r (#145)

[Andrew Ilijic]

Hi Rob,

> Hey Andrew: you wanted todo items, here's a fresh one if it looks interesting. :)
I read through the chain of emails and realized you were only referring to
`realpath` & `readlink`, lol.

To restate the request, you want the options `-LPefmnqsz --relative-to
--relative-base` implemented for `realpath`. These options are the
same or similar
to the `readlink` options, and so we should reuse the existing code.
Can you give
me an example of other merged commands?

Looking at the dispatch code that runs on the first argument, it seems
we need the
function `realpath_main`. My first thought is, use realpath_main as a wrapper,
manipulate the flags that need it, and then call into and use as much of
`readlink` as we can, does that make sense?

~Andrew