RFR: 8333775: Small improvement to outputStream auto-indentation mode

[Thomas Stuefe]

Almost trivial enhancement.

[JDK-8333211](https://bugs.openjdk.org/browse/JDK-8333211) added automatic indentation. Some changes to complement that:

- let outputStream::set_autoindent() return the old value for later restoration
- add an RAII object to enable autoindent and restore the old state when leaving.

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Commit messages:
 - JDK-8333775-Small-improvement-to-outputStream-auto-indentation-mode

Changes: https://git.openjdk.org/jdk/pull/19592/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19592&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8333775
  Stats: 35 lines in 5 files changed: 20 ins; 9 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/19592.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19592/head:pull/19592

PR: https://git.openjdk.org/jdk/pull/19592