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

[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 incrementally with one additional commit since the last revision:

  8293331: Reinstate #include "jni.h"

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Changes:
  - all: https://git.openjdk.org/jdk/pull/10154/files
  - new: https://git.openjdk.org/jdk/pull/10154/files/a2da4deb..32d042d4

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10154&range=06
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10154&range=05-06

  Stats: 3 lines in 3 files changed: 3 ins; 0 del; 0 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