RTLD_DEEPBIND: make lookup not just symbolic, but walk all refobj' DAGs

before starting the walk over the global list. Effectively we visit
needed objects first as well, instead of just the object itself.
This seems to better match the semantic offered by the glibc flag.

Reported by: kevans
PR: 275393
Reviewed by: kevans
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D42841