cli: fix top level --help combined with other options

If the top level --help is combined with other options, help
fails. For example:

    $ notmuch --version --help

    Sorry, --help is not a known command. There's not much I can do to help.

Fix this by adjusting argc and argv appropriately. The help command
ignores argv[0] anyway, so we don't have to set it to "help".