Use OpenSSL 1.1 API to access DSA and RSA internals

Use OpenSSL 1.1 API to access DSA and RSA internals

In LibreSSL 3.5, DSA and RSA internals will become opaque, so they can
no longer be accessed directly and the libunbound build will break. The
required API, DSA_set0_pqg(), DSA_set0_key() as well as RSA_set0_key(),
has been available since LibreSSL 2.7, so this change should not affect
any users.

From Theo Buehler.