[MI] -break-delete with several breakpoints

[Vladimir Prus <vladimir@codesourcery.com>]

I've noticed that right now, both -break-delete and -break-disable
commands accept several breakpoint ids, like:

	-break-disable 1 2 3

This behaviour comes almost by accident, and is not documented anywhere.
The question is -- should we document it and add tests, or should we
declare this behaviour does not exist?

I think that most of the time, making use of this behaviour will require
explicit code in the frontend, and the question is if that makes sense.
Say, for -var-update accepting a list of variable object might be good idea,
since the number of variable object can be significant, and they are updated
on each step. For deleting and disabling breakpoints, I'm actually not sure.
Typically, there are few breakpoints and wholesale delete is not common,
so it's not worth optimizing for.

Opinions?

- Volodya