lib: simplify interface of setsockopt_multicast_ipv4().

lib: simplify interface of setsockopt_multicast_ipv4().

* sockopt.[ch] (setsockopt_ipv4_multicast): ifindex is now mandatory (all
  non-ancient OSes can use it anyway), and if_addr parameter (the address
  of the interface) is now gone. (setsockopt_ipv4_multicast_if):
  IP_MULTICAST_IF processing moved to this new function

* ospf_network.c (ospf_if_add_allspfrouters, ospf_if_drop_allspfrouters,
  ospf_if_add_alldrouters, ospf_if_drop_alldrouters, ospf_if_ipmulticast),
  rip_interface.c (ipv4_multicast_join, ipv4_multicast_leave,
  rip_interface_new): adapt to the new interface