Preserve errno on dlopen

Preserve errno on dlopen

For the same reasons as in the previous commit we must preserve errno
across dlopen calls. Some implementations (e.g. solaris) do not preserve
errno even on a successful dlopen call.

Fixes #6953

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7680)

(cherry picked from commit 3cb4e7dc1cf92022f62b9bbdd59695885a1265ff)