RFR: 8294399: (ch) Refactor some methods out of sun.nio.ch.UnixFileDispatcherImpl [v4]

[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: Move UnixDispatcher_init() before the other JNI methods

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Changes:
  - all: https://git.openjdk.org/jdk/pull/10434/files
  - new: https://git.openjdk.org/jdk/pull/10434/files/b66ff40e..ab83200d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10434&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10434&range=02-03

  Stats: 24 lines in 1 file changed: 12 ins; 12 del; 0 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