DH, DSA, EC_KEY: Fix exporters to allow domain parameter keys

DH, DSA, EC_KEY: Fix exporters to allow domain parameter keys

The export-to-provider functions for DH, DSA and EC_KEY assumed that a
public key is always present, and would fail if not.  This blocks any
attempt to export a key structure with only domain parameters.

While fixing this, we also modify the selection declaration to
evp_keymgmt_import() to be more adaptive, the diverse selection bits
are now added when the corresponding data is added to the OSSL_PARAM
array.

Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/11374)