[pdftex] \ignoreprimitiveerror implemented

[Karl Berry]

For the record: per Frank's request to Thanh (private mail) last July,
Thanh has implemented a new \ignoreprimitiveerror bitwise
parameter. It's enabled with -etex (which is set by default when
building the formats, etc.)

If the value is 1, the \vsplit error
  print_err("Infinite glue shrinkage found in box being split");
is changed to a warning. All other values/bits are reserved (and
currently do nothing).

I merged the change, using the same primitive name, into XeTeX. There
appeared to be no easy way to make an engine-independent change file.
I expect it is possible, but I gave up after a couple of failures.'
Other things to do.

I asked Luigi(+ Hans) to implement it for LuaTeX and the JP devs to
implement it for euptex. Those engines will probably have different
interfaces. tbd. --thanks, karl.