DLINKLIST: Add DLIST_FOR_EACH_SAFE macro
    
    This macro, as DLIST_FOR_EACH, iterates over the whole list. The main
    difference between both is that in the _SAFE version the pointer to the
    next list node is stored, allowing us to delete the current node safely.
    
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
    
    Reviewed-by: Sumit Bose <sbose@redhat.com>
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>