Obsoleting more progressively

[Stefan Monnier]

`make-obsolete' and friends are very useful to let us get rid of old
features, but even after 10 years of something being declared obsolete,
experience shows it's still in use "out there".
So I'd like to be able to make things "more obsolete" (i.e. create
a second level of obsolescence) before we actually remove them.

I can think of 2 ways to do implement that second level of obsolescence:
- Add warnings at runtime when obsolete stuff is used.
  for functions, commands and macros, make-obsolete that's reasonably
  easy to do; for variables it's more difficult.
  For hooks, we could let add-hook check the obsolescence property and
  emit a warning, and similarly for a few difference cases, but for
  the primitive get&set operations, this is not an option.
- Actually remove the function/variable from the non-released code.
  I.e. remove/deactivate the functions/variables from trunk during
  development but put them back in when we start pretesting.

Any thoughts?

  
        Stefan