Handle more keywords without argument

This "silently" introduces usage-hints for keywords. I'm not entirely happy with it, because this information is only available via the email-interface, and it is coupled with the error-message. It probably should better be split off of the error-message. Any maybe it should be put into the online-documentation and the emails should contain a link?

Either way, with this patch we gain proper handling of missing arguments for some more keywords.

x-get-key is not included because it has its own issue, which might produce a fix for a lack of arguments (#353).