nix --help: Group commands

src/libutil/ansicolor.hh

src/libutil/args.cc

src/libutil/args.hh

src/nix/add-to-store.cc

src/nix/cat.cc

src/nix/command.hh

src/nix/copy.cc

src/nix/dev-shell.cc

src/nix/doctor.cc

src/nix/dump-path.cc

src/nix/edit.cc

src/nix/eval.cc

src/nix/hash.cc

src/nix/log.cc

src/nix/ls.cc

src/nix/main.cc

src/nix/make-content-addressable.cc

src/nix/optimise-store.cc

src/nix/path-info.cc

src/nix/ping-store.cc