[tex-k] Reproducible builds using pdftex

[Karl Berry]

I'm sure I messed up somewhere, but I just committed Norbert's changes
(with minor modifications) to the texlive (r40889) and pdftex
repositories to implement SOURCE_DATE_EPOCH_TEX_PRIMITIVES.  (I didn't
like "FIXUP" in the name; for one thing, it's the opposite of a fix.) 
I also added some documentation to the pdftex manual and man page, and a
test.

Akira, I couldn't quite see if your changes conflicted with what I did.
Let me know if there are problems with your Windows compilation.

Please try it out if you can, and if it seems ok, I'll ask the TL
builders to make yet another final rebuild in a day or two.

Thanks,
Karl