On platforms where pointers are larger than 64-bits, struct statsblob
may be harmlessly padded out such that opaque always has some included
space. Make the assertion more general by compating to the offset of
opaque rather than the size of sizeof(struct statsblob).
truss: centralize pointer-constructing casts.
Arguments pointers are passed around as integrer types and need
to be converted to pointers when performing ptrace IO operations such as
in get_struct(). Move the casts into get_struct() and friends and take
a uintptr_t argument rather than a void *.
Add a print_pointer function that takes a uintptr_t and uses a single
case plus "%p" to print pointers.