Add gss_wrap_size_limit() like functionality

[troelsarvin]

I'm trying to use the ldap3 crate, and I'm running into some problems there. When I dive into the ldap3 code, I see this comment:

        // FIXME: the max_size constant is taken from OpenLDAP GSSAPI code as a fallback
        // value for broken GSSAPI libraries. It's meant to serve as a safe value until
        // gss_wrap_size_limit() equivalent is available in cross-krb5.

So I hope gss_wrap_size_limit-like functionality can be added to cross-krb5. The following code (which was subsequently reverted) was maybe an attempt at doing that? 90796a0
If so, I perhaps it could be tried again?