automatically retrieve symbols from MS symbol server

_From [derek.br...@gmail.com](https://code.google.com/u/117968039472581148324/) on December 10, 2010 17:57:59_

PR 463897

several features to improve winsyms.c, the Windows addr2line app that uses
dbghelp.dll from PR 463895:
- until we have full paths from DR (issue #138):
have DrMem log all .dll files opened and then add those dirs to search path?
- if we pass in a handle to target process (have a "pid=" input), this will
load symbols for all its modules, so we could avoid having to load them
one by one or to guess their paths. but still have to handle modules
loaded later. and no guarantee process still alive by time sideline
symbol processor accesses it.
- support symbol servers, so users can use downloaded Windows system pdbs
- be more robust about handling failures packing in loaded modules. E.g.,
today we will probably fail if passed two .exe's (non-relocatable). we
should query the address space to find more appropriate places to load,
and potentially unload existing modules (they'll be re-loaded on demand
later).
- : have winsyms take in a query and returns whether just has
export symbols?

_Original issue: http://code.google.com/p/drmemory/issues/detail?id=143_