RFR: 8318026: jcmd should provide access to low-level JVM debug information [v4]

[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 three additional commits since the last revision:

 - Usage correction
 - Help to clarify this is VM inspection.  Comment to relate source to debug.cpp.
 - jcheck trailing whitespace

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/17655/files
  - new: https://git.openjdk.org/jdk/pull/17655/files/cacbca27..1d1e20e2

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17655&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17655&range=02-03

  Stats: 7 lines in 3 files changed: 1 ins; 0 del; 6 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