Multicasts fix for OSX

[mwilniewiec]

Fix for #244

SIOGIFINDEX doesn't exist on OSX and no interface index was retrieved.
This was the under the hood problem of join_multicast_group() failure.

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[mwilniewiec]

I signed it!

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CLAs look good, thanks!

[bukepo]

Shall we use just if_nametoindex? It seems to be portable.

[mwilniewiec]

Im using osx so i made a safe fix. True it seems that if_nametoindex is available on all platforms but it would be nice to verify that first.