provide a command line option like --list-tools, it can help some shell completion of `valgrind --tool=<TAB>` <https://github.com/zsh-users/zsh/blob/master/Completion/Linux/Command/_valgrind#L259>
--help outputs tool-selection option, with default in [ ]: --tool=<name> use the Valgrind tool named <name> [memcheck] available tools are: memcheck cachegrind callgrind helgrind drd massif dhat lackey none exp-bbv Maybe I could add --tool=help to also print the above message.