crypto/engine/eng_devcrypto.c: open /dev/crypto only once

We opened /dev/crypto once for each session, which is quite unnecessary.
With this change, we open /dev/crypto once at engine init, and close
it on unload.

Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
(Merged from https://github.com/openssl/openssl/pull/7506)

(cherry picked from commit 458c7dad)