Use PointerHolder in several places where manually memory allocation
and deallocation were being used. This helps to protect against memory
leaks when exceptions are thrown in surprising places.
There were a few places in the code that were checking that a pointer
wasn't null before deleting it, even though C++ has always allowed
delete 0. Most of the code did not perform these checks.
Significantly improve the code's use of off_t for file offsets, size_t
for memory sizes, and integer types in cases where there has to be
compatibility with external interfaces. Rework sections of the code
that would have prevented qpdf from working on files larger than 2 (or
maybe 4) GB in size.
