[pdftex] Access value of output-directory

[Karl Berry]

    rely on an external tool and shell-escape.
    ...
    - http://tex.stackexchange.com/q/136012/10350
    - http://tex.stackexchange.com/q/294931/10350

Indeed, all of these examples use \write18.

My intuitive reaction is that once \write18 is involved, you have full
access to the entire system, and shouldn't (don't) need the engine's
help to get access to the value. Enrico showed one way.

I can agree it would be nice, in a way, but to me, the benefit of a
minor cleanliness doesn't justify a new primitive at this late date.

In another way, I also wouldn't want to do anything to encourage use of
\write18.

But, it's not up to me alone.  Thanh (and anyone else), what do you
think?

best,
karl