RFR: 8354464: Additional cleanup setting up native.encoding

[Stuart Marks]

Remove the #ifdef MACOSX from platform-independent System.c, which (on Mac) sets native.encoding from sprops.encoding or (on non-Mac Unix and Windows) sets native.encoding from sprops.sun_jnu_encoding. After this removal, the native.encoding property will be set from sprops.encoding and the sun.jnu.encoding property will be set from sprops.sun_jnu_encoding.

Change the windows java_props_md.c so that it initializes sprops.sun_jnu_encoding and then copies that value to sprops.encoding. Previously, sprops.encoding was initialized with data from other Windows APIs, but this value was ignored by platform-independent code, which was kind of confusing.

There are no changes to the Unix (Mac & non-Mac) platform-specific code. It already has code to set sprops.encoding and sprops.sun_jnu_encoding properly, including macOS-specific code to set the latter to UTF-8 unconditionally.

There should be no behavior changes on any platform.

I changed variable names in windows/java_props_md.c to make it clear that the values stored into them are unused. Of course there are other possibilities here, such as changing the interface to SetupI18nProps(). I'm open to suggestions.

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Commit messages:
 - Rename variables to make clear they're not used.
 - 8354464: Additional cleanup setting up native.encoding

Changes: https://git.openjdk.org/jdk/pull/24607/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24607&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8354464
  Stats: 17 lines in 2 files changed: 2 ins; 12 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/24607.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24607/head:pull/24607

PR: https://git.openjdk.org/jdk/pull/24607