[Toybox] ln implementation

[Andre Renaud]

Hi Elie,
> If you change
> 
> USE_LN(NEWTOY(ln, "fs", TOYFLAG_BIN))
> 
> into
> 
> USE_LN(NEWTOY(ln, "<2>2fs", TOYFLAG_BIN))
> 
> Then this equals the same as stating that two arguments (other than f s)
> are expected (see toys/link.c and lib/args.c for inspiration). Then you
> can omit null-pointer checking and the usage printing. On my system it
> then just behaves as follows:

Thanks for that - I haven't quite got to grips with the magic involved
in the USE_xx macros.

Regards,
Andre