UTIL: Add type-specific getsetters to sss_iobuf
    
    The KCM responder receives its input as unstructured data. To make the
    parsing easier, this commit adds several type-specific getsetters to the
    iobuf module.
    
    Reviewed-by: Michal Židek <mzidek@redhat.com>
    Reviewed-by: Simo Sorce <simo@redhat.com>