RFR: 8333775: Small improvement to outputStream auto-indentation mode [v2]

[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.

Thomas Stuefe has updated the pull request incrementally with one additional commit since the last revision:

  feedback johann

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/19592/files
  - new: https://git.openjdk.org/jdk/pull/19592/files/4d70ec07..9599ebaf

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=19592&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=19592&range=00-01

  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 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