We need static factory creation methods on java.nio.channels.SocketChannel and java.nio.channels.ServerSocketChannel which take a ProtocolFamily

public class SocketChannel {
    public static SocketChannel open(ProtocolFamily family) throws IOException
}

public class ServerSocketChannel {
    public static ServerSocketChannel open(ProtocolFamily family) throws IOException
}

together with supporting spi methods in SelectorProvider.

The behavior should be equivalent to the similar existing method on DatagramChannel, which is to support:

StandardProtocolFamily.INET => an IPv4 only channel

StandardProtocolFamily.INET6 => a dual stack IPv4/IPv6 channel on dual stack systems, and an IPv6 only channel on an IPv6 only system

This will also facilitate the support of Unix domain as another standard protocol family.

A prototype of this work is available in the JDK sandbox repo in the `JDK-8238770-branch` (note the different bug id).