RFR: JDK-8313804: JDWP support for -Djava.net.preferIPv6Addresses=system

[Liam Miller-Cushon]

Please consider this fix for [JDK-8313804](https://bugs.openjdk.org/browse/JDK-8313804), which adds support to JDWP for `-Djava.net.preferIPv6Addresses=system`. Previously it only handled `-Djava.net.preferIPv6Addresses=true` and `-Djava.net.preferIPv6Addresses=false`.

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Commit messages:
 - JDK-8313804: JDWP support for -Djava.net.preferIPv6Addresses=system

Changes: https://git.openjdk.org/jdk/pull/15796/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15796&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8313804
  Stats: 53 lines in 2 files changed: 45 ins; 0 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/15796.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15796/head:pull/15796

PR: https://git.openjdk.org/jdk/pull/15796