RFR(XS): 8146482: [TESTBUG] java/net/SocketOption/OptionTest should only use multicast capable interfaces for multicast tests

[Volker Simonis]

Hi,

can somebody please review this small test fix:

http://cr.openjdk.java.net/~simonis/webrevs/2016/8146482/
https://bugs.openjdk.java.net/browse/JDK-8146482

The java/net/SocketOption/OptionTest test chooses the first network
interface returned by NetworkInterface.getNetworkInterfaces() for
doing some tests on multicast sockets.

But it may be that the chosen interface does not even support
multicasting. So better iterate over all interfaces and only choose
one if NetworkInterface.supportsMulticast() returns true.

Thanks,
Volker