RFR: 8293331: Refactor FileChannelImpl into operating system-specific components

[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.

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Commit messages:
 - 8293331: Refactor FileChannelImpl into operating system-specific components

Changes: https://git.openjdk.org/jdk/pull/10154/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10154&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8293331
  Stats: 1294 lines in 13 files changed: 865 ins; 391 del; 38 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