7026376: (dc) DatagramChannel created without specifying protocol family fails to join IPv4 group

[Alan Bateman]

This is a small fix to the mulitcast support in DatagramChannel. If 
someone creates a DatagramChannel without specifying the protocol family 
then they get a DatagramChannel to an IPv6 socket if IPv6 is enabled. If 
they then attempt to join an IPv4 multicast group then it may succeed or 
may fail (the spec is clear that there is no guarantee that a channel to 
a socket in one protocol family can join a multicast group with an 
address that corresponds to another protocol family). When not supported 
then the exception today is not clear. The change here ensures that 
IllegalArgumentException is thrown with a clear message detail when not 
supported and for Linux, changes the implementation to use Net.join4 
rather than Net.join6 for this case. The webrev with the changes is here:
  http://cr.openjdk.java.net/~alanb/7026376/webrev/

Thanks,
Alan