RFR: 8278268 - (ch) InputStream returned by Channels.newInputStream should have fast path for FileChannel targets

[Markus KARG]

This sub-issue defines the work to be done to implement JDK-8265891 solely for the particular case of FileChannel.transferFrom(ReadableByteChannel), including special treatment of SelectableByteChannel.

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Commit messages:
 - Implementing 8278268

Changes: https://git.openjdk.java.net/jdk/pull/6711/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6711&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278268
  Stats: 72 lines in 2 files changed: 59 ins; 2 del; 11 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6711.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6711/head:pull/6711

PR: https://git.openjdk.java.net/jdk/pull/6711