Extend support for PSS key signatures by using the EVP_PKEY_RSA_PSS type
to distinguish them from PKCS1 signature types.

Allow setting of PSS signature algorithms using the string "PSS" or
"RSA-PSS".
Reviewed-by: NRichard Levitte <levitte@openssl.org>
Reviewed-by: NMatt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2301)