[jdk11u-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 are the differences from the original patch in order to get this backport working for 11u:

make/lib/Lib-jdk.net.gmk -- Updated this script to support building extnet for Windows.

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

src/java.base/windows/classes/java/net/PlainSocketImpl.java -- The Windows PlainSocketImpl class will need to have similar setOption/getOption methods as the Unix PlainSocketImpl class to support setting these extended socket options for Windows.

Backport passed against Tier 1 tests and was manually verified to be working on Windows.

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Commit messages:
 - Corrected copyright years
 - Backport f3ade388dac0b882e671462caa762138f44817fb

Changes: https://git.openjdk.org/jdk11u-dev/pull/2278/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=2278&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8308593
  Stats: 345 lines in 9 files changed: 312 ins; 0 del; 33 mod
  Patch: https://git.openjdk.org/jdk11u-dev/pull/2278.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev.git pull/2278/head:pull/2278

PR: https://git.openjdk.org/jdk11u-dev/pull/2278