util/memory: sanitization
    
     - moved private `struct mem_holder` out of public header
     - moved `sss_mem_attach()` function description to the declaration
     - changed `sss_mem_attach()` to return error code instead of a pointer
       as data it points to is not expected to be manipulated by a user
     - got rid of redundant `sdap_msg_attach()`
    
    Reviewed-by: Sumit Bose <sbose@redhat.com>