pk: add flags to force RSA-PSS salt length to match digest length

pk: add flags to force RSA-PSS salt length to match digest length

This adds a couple of flags to RSA-PSS signing and verification, to
enforce that the salt length matches the digest length.  That is not
only recommended in RFC 4055, but also mandated in RFC 8446 in the TLS
1.3 context.

Signed-off-by: Daiki Ueno <ueno@gnu.org>