This is a very gentle commit that just lays the groundwork in the
build system: it puts the include files to build libtor-app.a into
src/core, and to build the tor executable into src/app. The
executable is now "src/app/tor".
Add more explanation in doc/HACKING about how to read gcov output,
including a reference to the gcov documentation in the GCC manual.
Also add details about how our postprocessing scripts modify gcov
output.
