RFR: 8350703: Add standard system property stdin.encoding [v3]

[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

Stuart Marks has updated the pull request incrementally with one additional commit since the last revision:

  Yet another minor bit of wordsmithing.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/24738/files
  - new: https://git.openjdk.org/jdk/pull/24738/files/5357e504..c4bf316d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=24738&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=24738&range=01-02

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