integrate leak detection's zero stack with pattern mode

_From [zhao...@google.com](https://code.google.com/u/106321947286816917100/) on February 24, 2012 17:43:40_

DrMemory's leak detection zeros the stack on stack pointer changes to detect more memory leaks, e.g. malloc test case on linux.
Current pattern mode implementation makes it difficult to add stack-zero instrumentation.
Need figure out a way to do the zeroing.

_Original issue: http://code.google.com/p/drmemory/issues/detail?id=797_