Cache maskHash parameter

Store hash algorithm used for MGF1 masks in PSS and OAEP modes in PSS and
OAEP parameter structure: this avoids the need to decode part of the ASN.1
structure every time it is used.

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