Use `struct ip_mreqn' instead of `struct ip_mreq' for multicast
operations on 2.1/2.2 kernels. This allows passing of real interface
indexes instead of referencing interfaces by their IP addresses which
fails badly in presence of unnumbered interfaces.
Unfortunately, this structure is not visible with glibc 2.0 as it provides
its own networking headers :-( Both libc5 and glibc 2.1 should be OK.