Use relative paths instead of absolute paths when exporting a profile

Using relative paths should allow you to open a profile produced on a different machine than your own, but still use the source view, which is one of the most useful aspects of PProf.

Also, when loading a profile, `pprof` takes an option to set a source file path, which can be used to lookup the relative paths, which makes this more useful.

I think this should be as simple as calling relpath in a couple places.