JDK-8219143: jdb should support breakpoint thread filters

[Chris Plummer]

Hello,

Please review the following:

http://cr.openjdk.java.net/~cjplummer/8219143/webrev.00/
https://bugs.openjdk.java.net/browse/JDK-8219143

Normally when a breakpoint is set in jdb, it is set globally (all 
threads). JDI supports the ability to have the breakpoint be 
automatically filtered so it will only be delivered on a specified 
thread and ignored on all other threads. This change allows making use 
of that JDI feature from jdb. So instead of something like:

   stop at Foo:23

You can now do:

   stop at threadid 7 Foo:23

Where 7 is the threadID for the thread as seen in the output of the 
"threads" command. The format of the stop command is now:

    stop [go|thread] <at|in> [threadid <thread_id>] <location>

It is still fully backwards compatible.

As part of this change I also cleaned up the "stop" command parsing and 
error handling. It was kind of a mess, and was near impossible to add 
the "threadid" option until after I did much of the cleanup. I've also 
cleaned up the help output a lot, and added help for the "go" and 
"thread" options. One last change was to remove the distinction between 
"stop at" and "stop in", which was suggested already in a comment. They 
can be used interchangeably now. The changes in Commands.java are pretty 
much all just the above cleanup, except for the one short section with 
the 'Handle "threadid" modifier' comment.

thanks,

Chris