apt: Support old resolver as fallback

Not all C libraries implement the new resolver APIs e.g. musl, therefore
keep the old interfaces around and choose using __RES version macro
which is meant for this kind of work.

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Cc: Alexander Kanavin <alex.kanavin@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>