[MLton] mlnlffigen is awesome (was: bootstrap problem)

[Matthew Fluet]

> I would say that using ~/.mlton/mlb-path-map by default is fine.  I would
> vote for allowing multiple -mlb-path-map arguments, allowing later ones to
> override earlier ones.  The only question is: do we disable the use of
> ~/.mlton/mlb-path-map by the presence of any -mlb-path-map args?  If so,
> then you might need to do the weird
> 	-mlb-path-map /dev/null
> to indicate no variables at all.

Yeah, the /dev/null hack is why I would vote for no implicit 
~/.mlton/mlb-path-map.

Although, I could be convinced that having any explicit -mlb-path-map 
argument disables the implicit ~/.mlton/mlb-path-map would be good enough.
After all, one reason to use -mlb-path-map would be because you want to 
document the MLB path variables, in which case, an empty file is 
itself documentation.