improve accuracy of -leaks_only

_From [derek.br...@gmail.com](https://code.google.com/u/117968039472581148324/) on December 10, 2010 17:58:19_

PR 539395

PR 520916 added a solid leak-check-only mode that is pretty accurate but
there are some issues and room for improvement:
- issues w/ ignoring beyond-TOS w/ alternate stacks:
- if currently on alt sigstack will look beyond TOS on main stack
- if currently on main stack will look at whole alt stack
could intercept signal + sigreturn.
- are there uninit fields in kernel stack allocs for signal/Ki?
- use an interval tree of mallocs (despite perf cost: see PR 535568) or
some other way to improve accuracy and efficiency of locating live
mallocs in an unmapped arena

_Original issue: http://code.google.com/p/drmemory/issues/detail?id=183_