Rewrite CMS_SignerInfo_{sign,verify}()

Rewrite CMS_SignerInfo_{sign,verify}()

Convert to using one-shot signing and verification. This is simpler than
doing Init/Update/Final and necessary for Ed25519 support (RFC 8419). Use
a single exit idiom, don't reuse the same buffer for decoding and signing
and simplify a few other things.

ok jsing