RFR: 6539707: (fc) MappedByteBuffer.force() method throws an IOException in a very simple test

[Brian Burkhalter]

This change proposes to increase the number of retries of `FlushViewOfFile` in the Windows native implementation of `MappedByteBuffer.force()`, and to catch any exception thrown by the native `force()` and rethrow an `UncheckedIOException` with cause set to the intercepted exception. A sentence is added to the specification of `MappedByteBuffer.force()` regarding unspecified errors. The test from the issue description is revised to fail if `force()` throws an exception which is not an `UncheckedIOException`, or if it is an `UncheckedIOException` whose message indicates it was thrown by the Windows native implementation of `force()`.

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Commit messages:
 - 6539707: (fc) MappedByteBuffer.force() method throws an IOException in a very simple test

Changes: https://git.openjdk.java.net/jdk/pull/2636/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2636&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-6539707
  Stats: 90 lines in 3 files changed: 87 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2636.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2636/head:pull/2636

PR: https://git.openjdk.java.net/jdk/pull/2636