[tex-live] pdfetex the default engine

[Karl Berry]

    \pdfoutput is not the only primitive added by pdftex and not everybody
    checks for \pdfoutput. E.g. geometry does:

So, how about undefining all the primitives.

Existing code mostly assumes that latex/ do not have pdf primitives, and
pdflatex/pdftex do.   This does not seem like an unreasonable assumption
to me, although I agree that an ifpdf.sty-like-approach is what we
should recommend.

Even after fixing every single package on TL (doable, I suppose), it
will be painful for the users to break their existing documents when
they happen to use an old package with a new tex (happens all the time),
or if they test for \pdfoutput (or whatever) directly in their documents
-- which they do.  I've done it myself, many times.