Allow to specify UDPv4/6 multicast interface

Allow to specify UDPv4/6 multicast interface

Using a Posix function, this allows to specify outgoing/incoming
multicast interfaces, e.g.

  para_server --udp_target=224.0.1.38:8000 --udp_mcast_iface=eth1

or

  para_audiod -D -r 'ogg:udp -i 224.0.1.38 -I eth0'

The option simplifies multicast streaming, which is now possible without
having to set a multicast route when there is more than one network interface.

This option works unconditionally for UDPv6. For UDPv4 it is only enabled
on hosts that support `struct ip_mreqn', support for which is detected
via configure.

On OSes that do not support this struct, a warning message is printed;
while it is possible to add the same functionality also in those cases,
it would complicate the implementation and thus has been left out.