Set EVP_PKEY_CTX in SignerInfo

If we aren't setting public key parameters make EVP_PKEY_CTX available
in SignerInfo so PSS mode and parameters are automatically selected.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2177)