[pdftex] Please make the CreationDate, ModDate and ID field deterministic

[Karl Berry]

Akira, Nicolas -

Thanh is away for ~3 weeks.  He will review both the SOURCE_DATE_EPOCH
patch (which I suspect will be fine) and Nicolas's other comments when
he's back.

The current code is not setting anything in stone.  It was a first shot,
so we could try to garner feedback from some TeX people who chimed in on
the conversation (elsewhere).

Best,
Karl