RFR: 8357244: Move class declarations from attachListener_posix.cpp to header files

[Dmitry Cherepanov]

The patch suggests moving class declarations from `attachListener_posix.cpp` to header files

It's moved in `openjdk/crac` project ahead of upstream and this causes merge conflicts every time attachListener code changes (e.g. https://github.com/openjdk/crac/pull/224). It's therefore suggested that the changes are included in upstream.

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Commit messages:
 - attach listeners

Changes: https://git.openjdk.org/jdk/pull/25300/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25300&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8357244
  Stats: 279 lines in 3 files changed: 178 ins; 101 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/25300.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25300/head:pull/25300

PR: https://git.openjdk.org/jdk/pull/25300