RFR: 8293331: Refactor FileChannelImpl into operating system-specific components [v2]

[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 two additional commits since the last revision:

 - 8293331: Move OS-specific code to FileDispatcher; remove OS-specific FileChannelImpl classes
 - 8293331: correct an error in the AIX FileChannelImpl.c

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/10154/files
  - new: https://git.openjdk.org/jdk/pull/10154/files/1d9b50c7..4a3e5fc7

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10154&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10154&range=00-01

  Stats: 1682 lines in 17 files changed: 581 ins; 1062 del; 39 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