bug#2631: get-free-disk-space when default-directory doesn't exist

[Kevin Ryde]

Stefan Monnier <monnier@iro.umontreal.ca> writes:
>
> why not bind default-directory to dir?

I think that'd make it error out on a non-existant dir argument, where
currently it returns nil (from the df parsing code, and it looks like
from file-system-info in dosfns.c).

There's a few other places binding to "/", like man.el and message.el,
if it ought to be something else ...