improve app debugging under windbg, including when inside tool routines

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on September 22, 2011 16:03:45_

xref issue #532 : improve app debugging under gdb

this is the counterpart for windbg

for tools we will want additional capabilities: e.g., Dr. Memory supports
an "attach debugger on error" feature, but upon attaching, control is in
the middle of a sequence of Dr. Memory functions which were loaded by the
private loader, so it's not easy to figure out what's going on. if we can
we should support more than just plain-DR debugging.

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=559_