This patch implements MI support for multiexec. I attach my notes on design, as well
as patch. The patch also contains documentation updates.

The executive summary is:
* thread groups of types 'process' are redefined to mean inferior, and therefore
  can exist before process is started, and outlive the process.
* The --thread-group option, previously available for select MI commands, is now
  globally available. Therefore, things like:

       -file-exec-and-symbols --thread-group i1 foobar
  
  works.
* The --all option to MI exec commands now affects threads in all inferiors.

The important caveat is that multiexec MI is only really working in non-stop.
At least the --all option, in all-stops, runs into various core issues. It's not
presently known how many are there and whether they are fixable.

- Volodya