[pdftex] Access value of output-directory

[Johannes Burtscher]

Final comment Karl; I promise! :)

   Enrico showed one way.

He kind of did. But I think the whole point of a new primitive would be to
release the user of the duty to specify the output directory *twice* (please
correct me if you're not referring to https://goo.gl/gs3r5g). My experience is
that as soon as you have to change something at two places, you sooner or later
forget about one.

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

I am not fully convinced that it would encourage the use of \write18. For sure,
it would be convenient for a few *package authors*, but I don't think that it
would get a "normal" user to turn on \write18 any sooner.

   ... the benefit of a minor cleanliness doesn't justify a new primitive
   at this late date.

Fully understandable.

Thanks!
Johannes