RFR JDK-8066678: java.nio.channels.Channels cleanup

[Pavel Rappo]

Hi everyone,

Could you please review my change for JDK-8066678?

http://cr.openjdk.java.net/~prappo/8066678/webrev.00/

It's a minor cleanup. Main things fixed:

	* Substituted Channels.checkNotNull for Objects.Objects.requireNonNull
          (non-essential difference is the NPE message)
	* Javadoc fixes

Thanks
-Pavel