Fixes for C sizes (esp. for 32-bit)

It appears that in the majority of cases hsize_t and hssize_t will be
64-bit types, even on 32-bit platforms. This site:
http://mitran-lab.amath.unc.edu:8082/subversion/bearclaw/hdf/include32/H5public.h
has the include file for 32-bit, and it does seem to depend on
H5_HAVE_LARGE_HSIZET. Everything I can find suggests this is always 1
(true), but if there are cases where this is false then these sizes
should default to the memory sizes.