RFR [9] 8157166: Update spec to account for platforms that do not support multicast

[Chris Hegarty]

Some platforms do not support multicasting, but the current spec requires that
it is supported. It seems reasonable to relax this, somewhat, so that multicasting
is effectively optional on these platforms. 

java.net.MulticastSocket.joinGroup and java.nio.channels.MulticastChannel.join
method specifications should be updated to indicate that an exception will be
thrown if invoked on a platform that does not support multicasting.

http://cr.openjdk.java.net/~chegar/8157166/
https://bugs.openjdk.java.net/browse/JDK-8157166

-Chris.