Rfc6810 bis

pull request for reviewing code
Should be unnecessary but gcc prints a warning without the cast.

/home/nils/git/rtrlib/./rtrlib/rtr/packets.c:802:16: warning: passing argument 4 of 
‘rtr_store_prefix_pdu’ from incompatible pointer type
             if(rtr_store_prefix_pdu(rtr_socket, pdu, sizeof(struct pdu_ipv4), &ipv4_pdus, &ipv4_pdus_nindex, &ipv4_pdus_size) == RTR_ERROR)