RFR: 8293331: Refactor FileChannelImpl into operating system-specific components [v4]

[Brian Burkhalter]

> 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 with a new target base due to a merge or a rebase. The pull request now contains seven commits:

 - Merge
 - 8293331: Clean up FileChannelImpl
 - 8293331: Split FileDispatcherImpl into operating system-specific components
 - Merge
 - 8293331: Move OS-specific code to FileDispatcher; remove OS-specific FileChannelImpl classes
 - 8293331: correct an error in the AIX FileChannelImpl.c
 - 8293331: Refactor FileChannelImpl into operating system-specific components

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Changes: https://git.openjdk.org/jdk/pull/10154/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10154&range=03
  Stats: 2668 lines in 16 files changed: 1459 ins; 1188 del; 21 mod
  Patch: https://git.openjdk.org/jdk/pull/10154.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10154/head:pull/10154

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