lib{c,sys}: move auxargs more firmly into libsys

Continue to filter the public interface (elf_aux_info()), but entierly
relocate the private interfaces (_elf_aux_info(),
__init_elf_aux_vector(), and __elf_aux_vector) to libsys.

This ensures that rtld updates the correct (only) copy of
__elf_aux_vector. After 968a18975adc9c2a619bb52aa2f009de99fc9e24
updates were confused and __getosreldate was failing, causing
the system to fall back to compat compat12 syscalls in some cases.

Return to explicitly linking libc to libsys and link libthr with libc
and libsys (in that order).

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D43910