obsoleting the annotate level 2 interface

[Jim Blandy <jimb@redhat.com>]

GDB seems to support two different ways of doing detailed annotations
of its output for consumption by other programs: MI and 'set annotate
2'.  I don't think annotation level 2 has many active users, if any at
all.  It pervades GDB's code.  Would it make sense to put 'set
annotate 2' on the path to obsolescence?

Some background: the 'set annotate' command sets the
'annotation_level' variable.  There are only three distinguished
values for this variable:

0: nothing special, GDB behaves normally.
1: in source.c:line_info and stack.c:print_frame_info, when we print
   the source line, we print out something extra that helps Emacs pop
   up the source code in a window.
2 or greater: we enable around 250 calls to a variety of functions in
   annotate.c to mark and identify lots of things GDB prints.

I think we should keep level 1, since the standard Emacs GDB interface
uses it, and it's not very much code.

I'd like to see GDB dump level 2, since it duplicates MI, badly.  MI's
design ensures that whoever's trying to parse GDB's output gets
something that's well-formed, whereas annotate just sticks escape
codes into the outgoing stream of text.  This means that, if you
change the way anything prints, you may break an annotate level 2
client.  But to break an MI client, you actually have to change a
ui_out call, whose sole purpose is to produce output for clients to
read.  So MI is a much more maintainable approach, because it's easier
for people to see what they're doing.

If folks agree that annotate level 2 should go, we could:
- announce that annotate level 2 will be disabled in the release after
  next;
- in that release, disable the code, but leave it there, to see if
  anyone complains, and whether they can be persuaded to switch to MI;
  and
- in the release after that, if all goes well, remove the code to
  support annotation level 2.

Personally, I'd like to see Emacs switch from annotation level 1 to
MI, too; then we could get rid of annotation altogether.  But I think
it makes sense to tackle level 2 first, since I don't think it has
many users (if any).