Fix creating and validating EdDSA signatures

Revert parts of commit c3b8130f which
inadvertently broke creating and validating EdDSA signatures:

 1. EVP_DigestSignInit() returns 1 on success.

 2. EdDSA does not support streaming (EVP_Digest*Update() followed by
    EVP_Digest*Final()), only one shot operations.
27 jobs for fix-eddsa-signatures-creation-and-validation in 12 minutes and 58 seconds (queued for 5 seconds)