RFR: 8304438: jcmd JVMTI.agent_load should obey EnableDynamicAgentLoading [v9]

[Serguei Spitsyn]

> The VM option EnableDynamicAgentLoading was added in JDK 9, default true, to allow deployment to choose whether to allow agents to be loaded/started in the VM. The VM option does the right thing for tools using the Attach API but jcmd JVMTI.agent_load was missed. This should be fixed to disallow loading JVMTI agents when the EnableDynamicAgentLoading is false.
> 
> The CSR is:
> [JDK-8309250](https://bugs.openjdk.org/browse/JDK-8309250): jcmd JVMTI.agent_load should obey EnableDynamicAgentLoading
> 
> Testing:
>  - run new test `test/jdk/sun/tools/jcmd/TestJcmdNoAgentLoad.java`
>  - TBD: submit mach5 tiers 1-5 to make sure no new regressions are introduced

Serguei Spitsyn 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 ten additional commits since the last revision:

 - Merge
 - review: add real java agent to TestJcmdNoAgentLoad
 - review: add to TestJcmdNoAgentLoad default and enabled dynamic loading modes
 - Merge
 - review: use output.shouldHaveExitValue(0) in the test
 - review: use output.shouldContain()
 - move check for EnableDynamicAgentLoading to invoke_Agent_OnAttach
 - minor renaming in new test TestJcmdNoAgentLoad.java
 - 8304438: jcmd JVMTI.agent_load should obey EnableDynamicAgentLoading

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/14244/files
  - new: https://git.openjdk.org/jdk/pull/14244/files/cf12a0ba..ada88c6d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=14244&range=08
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14244&range=07-08

  Stats: 10489 lines in 228 files changed: 8499 ins; 1035 del; 955 mod
  Patch: https://git.openjdk.org/jdk/pull/14244.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14244/head:pull/14244

PR: https://git.openjdk.org/jdk/pull/14244