support functions enumeration in drsym

_From [zhao...@google.com](https://code.google.com/u/106321947286816917100/) on March 22, 2013 16:43:32_

For code coverage, we want to know which function instead of lines are not executed, so function enumeration should be nice.

It looks like dbghelp.dll does not have function enumeration, I guess we might build it on top of symbol enumeration.

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=1131_