Simplify opensslecdsa_fromlabel

Simplify opensslecdsa_fromlabel

The 'opensslecdsa_fromlabel()' function does not need to get the
OpenSSL engine twice to load the private and public key. Also no need
to call 'dst_key_to_eckey()' as the EC_KEY can be derived from the
loaded EVP_PKEY's.

Add some extra checks to ensure the key has the same base id and curve
(group nid) as the dst key.

Since we already have the EVP_PKEY, no need to call 'finalize_eckey()',
instead just set the right values in the key structure.