RFR: 8293331: Refactor FileChannelImpl into operating system-specific components [v3]

[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 incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains five additional commits since the last revision:

 - 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:
  - all: https://git.openjdk.org/jdk/pull/10154/files
  - new: https://git.openjdk.org/jdk/pull/10154/files/4a3e5fc7..1af425b8

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10154&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10154&range=01-02

  Stats: 60548 lines in 634 files changed: 31625 ins; 24471 del; 4452 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