libctf: archive: allow opening BTF dicts in archives (not for upstreaming)

libctf: archive: allow opening BTF dicts in archives (not for upstreaming)

BTF dicts are normally suppressed in archives, but it is possible
to create them with enough cunning.  If such an archive is
encountered, the BTF dicts in it have no parent name, which
means that ctf_arc_import_parent (used by ctf_dict_open_cached,
ctf_archive_next, and all the ctf_arc_lookup functions) fails
to figure out what parent to import, and fails.

Kludge around it by relying on our secret knowledge that ctf_link_write
always emits the parent dict into the archive first.  If no name is set,
import the parent dict for now.  (Before upstreaming, a new archive format
with a dedicated parent dict field will turn up, obviating this kludge.)