elf_getarhdr: Replace per-archive Elf_Arhdr storage with per-member storage

elf_getarhdr: Replace per-archive Elf_Arhdr storage with per-member storage

Currently each archive descriptor maintains a single Elf_Arhdr for the
current archive member (as determined by elf_next or elf_rand) which is
returned by elf_getarhdr.

A single per-archive Elf_Arhdr is not ideal since elf_next and elf_rand
can invalidate an archive member's reference to its own Elf_Arhdr.

Avoid this possible invalidation by storing each Elf_Arhdr in its
archive member descriptor.

The existing Elf_Arhdr parsing and storage in the archive descriptor
internal state is left mostly untouched.  When an archive member is
created with elf_begin it is given its own copy of its Elf_Arhdr from
the archive descriptor.

Also update tests/arextract.c with verification that each Elf_Arhdr
is distinct and remains valid after calls to elf_next that would
have previously invalidated the Elf_Arhdr.

Signed-off-by: Aaron Merey <amerey@redhat.com>