8231259: (dc) DatagramChannel::disconnect re-binds socket to the wildcard address (macOS)

[Alan Bateman]

I've put a new webrev with the changes for this issue here:
    http://cr.openjdk.java.net/~alanb/8231259/2/webrev/

This moves the dup method from DatagramChannelImpl to NativeDispatcher. 
DatagramChannelImpl was okay but Chris is right that it's more general 
purpose and could be useful elsewhere.

-Alan