add option to not consider malloc padding as unaddressable

_From [derek.br...@gmail.com](https://code.google.com/u/117968039472581148324/) on December 10, 2010 17:58:18_

PR 535566

today I have Dr. Memory reporting any access to memory beyond what was
requested as an unaddressable access error. however, applications on Linux
can use malloc_usable_size() to retrieve the actually usable space, which
includes any padding from aligning the asked-for size or from using a
larger available free slot. we should support that by at least providing a
global option to only report errors beyond the padding: a better solution
would be a per-allocation relaxation, which would likely be an API (xref PR
406756)

_Original issue: http://code.google.com/p/drmemory/issues/detail?id=182_