sanon: Fix export/import_cred mismatch

sanon: Fix export/import_cred mismatch

SANON cred export/import never worked correctly as the export function was
producing the wrong form of token, which was leading gss_import_cred() to
allocate more than 64MB of memory to parse the SANON exported credential.  The
recent change to reduce the default `max_alloc` of krb5_storage exposed this.