RFR: JDK-8313804: JDWP support for -Djava.net.preferIPv6Addresses=system [v7]

[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`.

Liam Miller-Cushon has updated the pull request incrementally with one additional commit since the last revision:

  Add more test cases

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Changes:
  - all: https://git.openjdk.org/jdk/pull/15796/files
  - new: https://git.openjdk.org/jdk/pull/15796/files/832854c2..bd019306

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15796&range=06
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15796&range=05-06

  Stats: 47 lines in 1 file changed: 44 ins; 0 del; 3 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