RFR: 8365807: (fs) Two-arg UnixFileAttributes.getIfExists should not use exception for control flow

[Brian Burkhalter]

In the Unix implementation of `sun.nio.fs`, change

1. `UnixFileAttributes.getIfExists(UnixPath,boolean)` to use the system call success status and `errno` for control flow; and
2. `UnixFileAttributes.getIfExists(UnixPath)` to simply return `UnixFileAttributes.getIfExists(UnixPath,true)`.

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Commit messages:
 - 8365807: (fs) Two-arg UnixFileAttributes.getIfExists should not use exception for control flow

Changes: https://git.openjdk.org/jdk/pull/26889/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26889&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8365807
  Stats: 39 lines in 3 files changed: 7 ins; 13 del; 19 mod
  Patch: https://git.openjdk.org/jdk/pull/26889.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26889/head:pull/26889

PR: https://git.openjdk.org/jdk/pull/26889