RFR: 8262520: Add CLHSDB command to connect to debug server [v3]

[Yasumasa Suenaga]

On Tue, 2 Mar 2021 22:31:22 GMT, Chris Plummer <cjplummer at openjdk.org> wrote:

> This pre-existing code in CLHSDB.java is a little concerning:
> 
> ```
> 127     private void doUsage() {
> 128         System.out.println("Usage:  java CLHSDB [[pid] | [path-to-java-executable [path-to-corefile]] | help ]");
> 129         System.out.println("           pid:                     attach to the process whose id is 'pid'");
> 130         System.out.println("           path-to-java-executable: Debug a core file produced by this program");
> 131         System.out.println("           path-to-corefile:        Debug this corefile.  The default is 'core'");
> 132         System.out.println("        If no arguments are specified, you can select what to do from the GUI.\n");
> 133         HotSpotAgent.showUsage();
> 134     }
> ```
> 
> First concern is that you have not added "debug server" support to it. 2nd concern is the "If no arguments are specified, you can select what to do from the GUI" part. It looks like using the GUI you can launch the CLHSDB console, although I can't find in the source where/how that is actually done. In any case, i don't see any way to pass command arguments when you do that.

I just added the feature to connect to debug server, so I did not change `CLHSDB(String[] args)` and help message.
I think it should be changed when we add `--connect` option to `jhsdb clhsdb`. (I will work for it after this PR)

> 
> I'm just wondering how much of this CLHSDB support we want to keep beyond what the clhsdb command needs. Do we still want to support invoking CLHSDB.main() from the java command line?

I think we should remove `main()` from CLHSDB/HSDB to be honest because they are called from SALauncher, and we cannot call them directly because they are not exported in module-info.

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PR: https://git.openjdk.java.net/jdk/pull/2773