replace uses of U64_* and I64_* macros with their C99 stdint.h or inttypes.h equivalents
There are many macros in compat.h that try to portably handle 64-bit integer values in
scanf() calls. Now that we seem to require C99, we should use the equivalent macros from stdint.h or inttypes.h instead. For example,
U64_FORMAT is equivalent to
PRIu64 in inttypes.h.