When 'opensslecdsa_parse()' encounters a label tag in the private key
file, load the private key with 'opensslecdsa_fromlabel()'. Otherwise
load it from the private structure.
This was attempted before with 'load_privkey()' and 'uses_engine()',
but had the same flaw as 'opensslecdsa_fromlabel()' had previously,
that is getting the private and public key separately, juggling with
pointers between EC_KEY and EVP_PKEY, did not create a valid
cryptographic key that could be used for signing.
(cherry picked from commit 57ac70ad46)
4a36b6d918