RFR: 8294399: (ch) Refactor some methods out of sun.nio.ch.UnixFileDispatcherImpl [v2]

[Brian Burkhalter]

> Interpose a new class `sun.nio.ch.UnixDispatcher` between `sun.nio.ch.NativeDispatcher` and its current subclasses `sun.nio.ch.DatagramDispatcher` and `sun.nio.ch.SocketDispatcher`; refactor some methods out of `sun.nio.ch.UnixFileDispatcherImpl` into this updated hierarchy.

Brian Burkhalter has updated the pull request incrementally with one additional commit since the last revision:

  8294399: Correct copyright years

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Changes:
  - all: https://git.openjdk.org/jdk/pull/10434/files
  - new: https://git.openjdk.org/jdk/pull/10434/files/92dfca5f..a16bc10a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10434&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10434&range=00-01

  Stats: 3 lines in 3 files changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/10434.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10434/head:pull/10434

PR: https://git.openjdk.org/jdk/pull/10434