officially support post-process w/ USE_DRSYMS to redo callstacks with better symbols

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on June 09, 2011 08:35:35_

xref issue #143
xref issue #290
xref issue #388

On Linux there is already built-in support for re-processing callstacks. This issue is about Windows.

I want to avoid the complexities of having drmem download pdb files online
(DRi#450) or even in its front-end at startup.

xref Memcheck or Tsan: they require user to get symbols on his own.

Proposed model of acquiring symbols for users:
- use table for non-exported syscall wrapper #s (issue #388) so don't require
system dll pdbs (unless have static malloc or string routines)
- print warning that don't have pdbs
=> issue #143, DRi#448
- print sym+offs if don't have line#
=> issue #290: -symbol_offsets
- provide tool to download syms for all dlls in results.txt that need them.
tool also checks user32/kernel32 syscall #s vs drmem table.
- provide postprocess tool: have download-syms tool launch it.
- could have -batch-controlled msgbox pop up at end w/ notepad
that offers to run the command for cmdline-challenged users

_Original issue: http://code.google.com/p/drmemory/issues/detail?id=446_