RFR: 8318026: jcmd should provide access to low-level JVM debug information [v2]

[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 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 11 additional commits since the last revision:

 - Merge remote-tracking branch 'upstream/master' into 8318026_jcmd_VMdebug_command
 - Require UnlockDiagnosticVMOptions
 - Require UnlockDiagnosticVMOptionstest/hotspot/jtreg/serviceability/dcmd/vm/VMDebugTest.java
 - Tidy up the safety checks
 - Whitebox not required, just check option properties.
 - unnecessary parameter to find
 - Test update. Recognise ZGC oops differently.  Formatting.
 - typo
 - Separate is_good_oop... method to avoid changing existing asserts.
 - More oop safety
 - ... and 1 more: https://git.openjdk.org/jdk/compare/add19854...f57f7274

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/17655/files
  - new: https://git.openjdk.org/jdk/pull/17655/files/9d127e32..f57f7274

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17655&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17655&range=00-01

  Stats: 58500 lines in 2215 files changed: 25779 ins; 14954 del; 17767 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