Rework elf_aux_info for c18n

[bsdjhb]

Not all auxv entries are privileged (e.g. it probably is fine to permit reading scalar values like AT_HWCAP), so it might be nice to have separate functions for scalar vs pointer values (e.g. Linux-style getauxval() for scalars, and a getauxptr() for pointers). For the FreeBSD API, perhaps elf_aux_info(3) could only return scalar values, and a new elf_aux_info_tcb(3) or the like could return privileged values.