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

[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.

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Commit messages:
 - 8294399: (ch) Refactor some methods out of sun.nio.ch.UnixFileDispatcherImpl

Changes: https://git.openjdk.org/jdk/pull/10434/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10434&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294399
  Stats: 231 lines in 9 files changed: 158 ins; 60 del; 13 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