RFR: 8331195: Improve com.sun.net.httpserver.HttpExchange usability

[Michael McMahon]

I'd like to return to this issue with a new PR that addresses the usability issue minimally. Namely to define numeric constants
for the counter-intuitive responseLength values in `HttpExchange.sendResponseHeaders`. The apidoc then refers to the constant names rather than the numeric values.

I've updated the implementation and some tests (though not all yet) to use the constant names. I'm open to suggestions on the names themselves. Once that is agreed, I'll update the remaining tests and any other call sites in the implementation.

Thanks,
Michael

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Commit messages:
 - implementation and test
 - Merge branch 'master' into httpserver-8368955
 - doc update

Changes: https://git.openjdk.org/jdk/pull/28132/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28132&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8331195
  Stats: 60 lines in 9 files changed: 29 ins; 3 del; 28 mod
  Patch: https://git.openjdk.org/jdk/pull/28132.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28132/head:pull/28132

PR: https://git.openjdk.org/jdk/pull/28132