RFR: 8304438: jcmd JVMTI.agent_load should obey EnableDynamicAgentLoading [v3]

[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.
> 
> 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 incrementally with one additional commit since the last revision:

  move check for EnableDynamicAgentLoading to invoke_Agent_OnAttach

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/14244/files
  - new: https://git.openjdk.org/jdk/pull/14244/files/829a0e9a..be0ec0c9

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=14244&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14244&range=01-02

  Stats: 17 lines in 4 files changed: 6 ins; 9 del; 2 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