Allow usage on the first line of help output

[fitzsim]

Hi libre-man,

Thanks for maintaining unix-opts.

I'd like usage to go on the first line of --help output, and also to be able to specify a brief description of the tool just after the usage line, like GNU grep and other tools do.

This pull request does those two things.

If the API user wants a blank line between the prefix and usage lines, they can pass (format nil "this prefix~%") instead of "this prefix". I think that's better than hard-coding a terpri before the usage line, which doesn't give the option of putting usage on the first line.

Thanks, fitzsim