asynch signal arrives when in client callout: no bound on delivery

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on April 04, 2012 11:44:07_

DR only unlinks when can identify the fragment: else it assumes in DR and
so will see pending flag in dispatch.
one solution is to store the fragment prior to any clean call but we should search for a more performant solution.

(all the code on identifying dstack and looking for ntdll code, etc. is for
native threads for suspend-all)

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=717_