RFR: 8293502: (fc) FileChannel::transfer methods fail to copy /proc files on Linux

[Brian Burkhalter]

Files in `/proc` advertise a size of zero for which direct and mapped transfers to not work. For this case, fall back to arbitrary channel transfer.

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Commit messages:
 - 8293502: (fc) FileChannel::transfer methods fail to copy /proc files on Linux

Changes: https://git.openjdk.org/jdk/pull/10267/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10267&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8293502
  Stats: 205 lines in 2 files changed: 191 ins; 0 del; 14 mod
  Patch: https://git.openjdk.org/jdk/pull/10267.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10267/head:pull/10267

PR: https://git.openjdk.org/jdk/pull/10267