add post-mangling last-shot trace callback

_From [derek.br...@gmail.com](https://code.google.com/u/117968039472581148324/) on June 06, 2009 12:34:53_

this was PR 306163: add post-mangling last-shot trace callback

the old MIT API's trace callback was post-mangling. as part of our API
simplification we changed it to present application code. while complex
and less robust internally the API is much more straightforward that way.
however, advanced users may want the old post-mangling event, which allows
more flexible tweaking of trace code.

if want client to have access to our mangling: have separate event, since a
separate use case
- add is_DR_mangling() and tell them if they muck w/ our mangling
they have to restore the state.
- will our internal assumptions be messed up?
pattern matching: decode_fragment() looks for spills and short-cti pattern
=> strictly addition to API so we can punt for now

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=156_