RFR: 8318026: jcmd should provide access to low-level JVM debug information [v8]

[Kevin Walls]

> Introduce the jcmd "VM.inspect" to implement access to detailed JVM object information.
> 
> Not recommended for live production use.  Requires UnlockDiagnosticVMOptions and not included in jcmd help output, to remind us this is not a general-purpose customer-facing tool.

Kevin Walls has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains 20 additional commits since the last revision:

 - Change to jcmd VM.inspect
 - Merge remote-tracking branch 'upstream/master' into 8318026_jcmd_VMdebug_command
 - Test update
 - Show description if unknown subcommand.
 - Remove unnecessary 'events' subcommand.
 - Usage correction
 - Help to clarify this is VM inspection.  Comment to relate source to debug.cpp.
 - jcheck trailing whitespace
 - Test update omitted from previous commit.
 - Merge remote-tracking branch 'upstream/master' into 8318026_jcmd_VMdebug_command
 - ... and 10 more: https://git.openjdk.org/jdk/compare/872dc5a5...739bcbfa

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/17655/files
  - new: https://git.openjdk.org/jdk/pull/17655/files/3f566649..739bcbfa

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17655&range=07
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17655&range=06-07

  Stats: 458556 lines in 4892 files changed: 36637 ins; 91690 del; 330229 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