JDK-8177064: jcmd help command should not require the process identification

[Gary Adams]

Two years ago a request was made to allow

    jcmd help

to be interpretted as a request for help from the current jcmd process
rather than requiring a separate target process to be involved.

Some attempts were made to close the issue, because the
command was not documented to work that way.
It was also pointed out that the help from the running jcmd
might not match the cmds available from a subsequent
request to a different process possibly running a different version
of the vm.

As proof of concept exercise this webrev shows a minimal set of
changes that could support

   jcmd self help

Before fleshing out additional changes in documentation and testing,
I'd like to know if this is still a worth while enhancement. Also,
I'd like to know more about the history of the checks preventing self 
attach.

   Webrev: http://cr.openjdk.java.net/~gadams/8177064/webrev/index.html
   Issue: https://bugs.openjdk.java.net/browse/JDK-8177064