Additonally:
- remove "-s", it will not be used often
- remove --accountkey (former --authkey), as it is not used
- CLI_DEFAULTS: cert_path -> auth_cert_path, chain_path -> auth_chain_path
- remove remaining "LetsEncrypt" error prefixes (fixes#487).
- HeplfulParser:
- call add_plugin_args from outside (this makes sure plugins are
displayed last in --help all)