This function intialises an EVP_PKEY to contain a provider side internal
key.

We take the opportunity to also document the older EVP_PKEY_set_type()
and EVP_PKEY_set_type_str().

Reviewed-by: Matt Caswell <[email protected]>
(Merged from openssl#11375)

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 openssl#11366

Reviewed-by: Matt Caswell <[email protected]>
(Merged from openssl#11375)

Reviewed-by: Matt Caswell <[email protected]>
(Merged from openssl#11375)

Downgrading EVP_PKEYs from containing provider side internal keys to
containing legacy keys demands support in the EVP_PKEY_ASN1_METHOD.

This became a bit elaborate because the code would be almost exactly
the same as the import functions int EVP_KEYMGMT.  Therefore, we end
up moving most of the code to common backend support files that can be
used both by legacy backend code and by our providers.

Reviewed-by: Matt Caswell <[email protected]>
(Merged from openssl#11375)

Provider KEYMGMT functions can handle domain parameters as well as
"other" parameters (the cofactor mode flag in ECC keys is one of
those).  The public EVP functions EVP_PKEY_copy_parameters(),
EVP_PKEY_missing_parameters(), EVP_PKEY_cmp_parameters() and
EVP_PKEY_cmp() tried to handle all parameters, but looking back at
EVP_PKEY_ASN1_METHOD code (especially crypto/ec/ec_ameth.c), it turns
out that they only need to concern themselves with domain parameters.

Reviewed-by: Matt Caswell <[email protected]>
(Merged from openssl#11375)