RFR: 8318026: jcmd should provide access to low-level JVM debug information [v6]

[Kevin Walls]

> Introduce the jcmd "VM.debug" to implement access to a useful set of the established debug.cpp utilities, with "jcmd PID VM.debug subcommand ...".
> 
> Not recommended for live production use.  Calling these "debug" utilities, and not including them in the jcmd help output, is to remind us they are not general customer-facing tools.

Kevin Walls has updated the pull request incrementally with one additional commit since the last revision:

  Show description if unknown subcommand.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/17655/files
  - new: https://git.openjdk.org/jdk/pull/17655/files/6ba1c909..98a46d09

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17655&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17655&range=04-05

  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/17655.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17655/head:pull/17655

PR: https://git.openjdk.org/jdk/pull/17655