RFR: 4619075: (ch) newChannel() should return Gathering/ScatteringByteChannel

[Brian Burkhalter]

Modify `Channels.newChannel(InputStream)` and `Channels.newChannel(OutputStream)` to return `ScatteringByteChannel` and `GatheringByteChannel`, respectively.

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Commit messages:
 - 4619075: (ch) newChannel() should return Gathering/ScatteringByteChannel

Changes: https://git.openjdk.java.net/jdk/pull/5922/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5922&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-4619075
  Stats: 449 lines in 2 files changed: 437 ins; 0 del; 12 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5922.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5922/head:pull/5922

PR: https://git.openjdk.java.net/jdk/pull/5922