EVP: Downgrade keys rather than upgrade

EVP: Downgrade keys rather than upgrade

Upgrading EVP_PKEYs from containing legacy keys to containing provider
side keys proved to be risky, with a number of unpleasant corner
cases, and with functions like EVP_PKEY_get0_DSA() failing
unexpectedly.

We therefore change course, and instead of upgrading legacy internal
keys to provider side internal keys, we downgrade provider side
internal keys to legacy ones.  To be able to do this, we add
|import_from| and make it a callback function designed for
evp_keymgmt_export().

This means that evp_pkey_upgrade_to_provider() is replaced with
evp_pkey_downgrade().

EVP_PKEY_copy_parameters() is the most deeply affected function of
this change.

Fixes #11366

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/11375)