Add function coverage to drcov

From zhao...@google.com on April 29, 2014 22:34:31

Suggestions from Anand: while 100% line-coverage likely indicates 100% function-coverage as well, for components that have less than 100% line-coverage, knowing the function-coverage numbers helps to more quickly identify what functions should be prioritised for additional tests.

Original issue: http://code.google.com/p/dynamorio/issues/detail?id=1435