RFR: 8350703: Add standard system property stdin.encoding

[Stuart Marks]

* Windows and Unix: set sprops.stdin_encoding if connected to a console
* Add specs for stdin.encoding
* Adjust specs to change "undefined" to "unspecified"
* Rewrite System.in spec to refer to new property and to clarify usage with classes that perform encoding
* Update property test

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Commit messages:
 - Even moar wordsmithing.
 - moar wordsmithing
 - wordsmith
 - Revise spec of System.in to mention stdin.encoding.
 - Fix typo stdout => stdin
 - 8350703: first cut at stdin.encoding

Changes: https://git.openjdk.org/jdk/pull/24738/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24738&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8350703
  Stats: 48 lines in 7 files changed: 30 ins; 1 del; 17 mod
  Patch: https://git.openjdk.org/jdk/pull/24738.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24738/head:pull/24738

PR: https://git.openjdk.org/jdk/pull/24738