RFR: 8293331: Refactor FileDispatcherImpl into operating system-specific components [v6]

[Alan Bateman]

On Wed, 21 Sep 2022 00:49:50 GMT, Brian Burkhalter <bpb at openjdk.org> wrote:

>> Break up `sun.nio.ch.FileChannelImpl` in a manner similar but not identical to the refactoring of `sun.nio.fs.UnixFileSystem` performed in #10093.
>
> Brian Burkhalter has updated the pull request incrementally with one additional commit since the last revision:
> 
>   8293331: Remove unneeded C header files

The update removed the include of jni.h but JNI types are used. They get included by the javac generated header files but might be clearer to not remove it.

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PR: https://git.openjdk.org/jdk/pull/10154