Handle --help for all commands in the OCaml

The output is identical to the Python for all commands, except that:

- The Python incorrectly gave a --xml option for "add"
- The Python incorrectly missed --offline for "import" (to prevent
  downloading keys)
- Some meta vars have been renamed for clarity (e.g. --main=MAIN becomes
  --main=PATH).
- The order of the options is different.