KCM: request parsing and sending a reply
    
    Implements parsing the KCM client request into per-client buffers and
    sending a response for both the failure case and for success.
    
    The protocol is documented at:
        http://k5wiki.kerberos.org/wiki/Projects/KCM_client
    
    Several places don't use the sss_iobuf structure, because they don't
    parse variable-length data from the buffer and it's much more efficient
    to just allocate the needed request and reply structure on the stack.
    
    Reviewed-by: Michal Židek <mzidek@redhat.com>
    Reviewed-by: Simo Sorce <simo@redhat.com>