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.
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