tracediff: Rename --diff=TOOL option to --tool=TOOL.

More intuitive (I was always mistyping it) as it matches valgrind.
profileshader: Update script for changes in profile output.

Make the script smart enough to automatically adapt to future
formmating changes in profile output.
profileshader: Allow to specify which field to group on.

It allows, for example, to group results by function name, instead of
program.