RFR: 8293331: Refactor FileDispatcherImpl into operating system-specific components [v7]

[Tyler Steele]

On Wed, 21 Sep 2022 15:00:44 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: Reinstate #include "jni.h"

The tests are in progress :-)

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