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)
Status Job ID Name Coverage
  Precheck
passed precheck:debian:sid:amd64 #63247
amd64 docker linux

00:00:17

 
  Build
passed build:centos:centos6:amd64 #63248
amd64 docker linux

00:02:26

passed build:centos:centos7:amd64 #63249
amd64 docker linux

00:02:32

passed build:clang:debian:stretch:amd64 #63250
amd64 docker linux

00:01:45

passed build:clang:debian:stretch:i386 #63254
i386 docker linux

00:01:46

passed build:debian:jessie:amd64 #63251
amd64 docker linux

00:01:34

passed build:debian:sid:amd64 #63253
amd64 docker linux

00:01:39

passed build:debian:sid:i386 #63255
i386 docker linux

00:01:43

passed build:debian:stretch:amd64 #63252
amd64 docker linux

00:01:41

passed pkcs11:build:debian:sid:amd64 #63256
amd64 docker linux

00:01:42

 
  Test
passed install:debian:sid:amd64 #63270
amd64 docker linux

00:02:12

passed pkcs11:systemtest:debian:sid:amd64 #63272
amd64 docker linux

00:04:45

passed pkcs11:unittest:debian:sid:amd64 #63271
amd64 docker linux

00:00:47

passed systemtest:centos:centos6:amd64 #63264
amd64 docker linux

00:04:57

passed systemtest:centos:centos7:amd64 #63265
amd64 docker linux

00:04:51

passed systemtest:debian:jessie:amd64 #63266
amd64 docker linux

00:04:45

passed systemtest:debian:sid:amd64 #63268
amd64 docker linux

00:05:05

passed systemtest:debian:sid:i386 #63273
linux docker i386

00:04:42

passed systemtest:debian:stretch:amd64 #63267
amd64 docker linux

00:05:00

passed unittest:centos:centos6:amd64 #63257
amd64 docker linux

00:00:53

passed unittest:centos:centos7:amd64 #63258
amd64 docker linux

00:00:49

passed unittest:clang:debian:stretch:amd64 #63262
amd64 docker linux

00:00:55

passed unittest:debian:jessie:amd64 #63259
docker linux amd64

00:00:50

passed unittest:debian:sid:amd64 #63261
amd64 docker linux

00:00:45

passed unittest:debian:sid:i386 #63263
i386 docker linux

00:00:55

passed unittest:debian:stretch:amd64 #63260
amd64 docker linux

00:00:56

failed systemtest:debian:sid:i386 #63269
i386 docker linux

00:05:28