[jdk17u-dev] RFR: 8308593: Add KEEPALIVE Extended Socket Options Support for Windows

[Terry Chow]

This is an unclean backport of [JDK-8308593](https://bugs.openjdk.org/browse/JDK-8308593), which provides support for the keepalive extended socket options on Windows.

The following changes were made in these files to resolve conflicts and issues:
1. make/modules/jdk.net/Lib.gmk -- I updated this script to support building extnet for Windows.

2. jdk.net/share/classes/jdk/net/ExtendedSocketOptions.java -- I added a case to support returning an instance of WindowsSocketOptions in PlatformSocketOptions.

Tier 1 tests were ran and they're passing along with the existing keepalive tests. I also manually tested and confirmed the patch to be working on Windows.

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Commit messages:
 - Backport f3ade388dac0b882e671462caa762138f44817fb

Changes: https://git.openjdk.org/jdk17u-dev/pull/1751/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=1751&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8308593
  Stats: 298 lines in 8 files changed: 266 ins; 0 del; 32 mod
  Patch: https://git.openjdk.org/jdk17u-dev/pull/1751.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev.git pull/1751/head:pull/1751

PR: https://git.openjdk.org/jdk17u-dev/pull/1751