apps/apps.c: initialize and de-initialize engine around key loading

Before loading a key from an engine, it may need to be initialized.
When done loading the key, we must de-initialize the engine.
(if the engine is already initialized somehow, only the reference
counter will be incremented then decremented)

Reviewed-by: Stephen Henson <steve@openssl.org>
(cherry picked from commit 49e476a5)