Make data file path search more reusable

Make data file path search more reusable

Instead of returning the path to share/top from ProgramContextInterface,
return the installation prefix.  Make it the responsibility of the
caller to append the relative path to the data files it needs.  Clean up
config.h defines related to this to make them more generic.

This makes it possible to reuse the same logic to find files also in
other locations than share/top.  DataFileFinder is not yet similarly
reusable, but that is a topic for another change.

Change-Id: I7c7fc3730417e71ff43d1b6fadf00a0eb465d794