Convert casted X509_INFO stacks to type-safe STACK_OF(X509_INFO).

PS: Feel free to move the IMPLEMENT_STACK_OF(X509_INFO) from
    crypto/asn1/x_info.c to any other place where you think it fits better.
    X509_INFO is a structure slightly spreaded over ASN.1, X509 and PEM code,
    so I found no definitive location for IMPLEMENT_STACK_OF(X509_INFO).  In
    crypto/asn1/x_info.c it's at least now bundled with X509_INFO_new() and
    friends.