RFR: 8282691: add jdb support for passing --enable-preview and --add-modules to the debuggee JVM

[Chris Plummer]

jdb has 3 types of arguments:

1. Those that are jdb specific, such as -attach, -launch, and -listconnectors
2. Those that are passed to the JVM used to run jdb. These are all prefixed with -J, and any valid JVM argument can be passed in this manner.
3. Those that are passed to the debuggee JVM when it is launched, such as -classpath, -D<property>, and any option that starts with -X (including -XX)

The problem with the 3rd group is that (other than for -D and -X), jdb will only pass through arguments that it recognizes, and that list is very limited. When you want to launch the debuggee with an argument that jdb doesn't recognize, you have no choice but to launch the debuggee yourself (using a separate command line and using the -agentlib:jdwp argument) and then tell jdb to attach to the debuggee process. It's much easier when you can just let jdb launch the debuggee, and our nsk/jdb testing currently relies on this feature.

This PR adds --enable-preview and --add-modules to the list of arguments that jdb recognizes and passes through to the debuggee JVM. These seem to be the two most glaring omissions, and are two that will likely be needed soon in order for loom nsk/jdb testing to work properly.

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Commit messages:
 - Added support for passing --enable-preview and --add-modules to the debuggee JVM.

Changes: https://git.openjdk.java.net/jdk/pull/7708/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7708&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8282691
  Stats: 18 lines in 4 files changed: 14 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7708.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7708/head:pull/7708

PR: https://git.openjdk.java.net/jdk/pull/7708