Fixed Urchin invocation problem in zsh and dash.  

The - currently - pointless invocations of Urchin itself from the various shells have been removed, pending the amendment to Urchin that will allow true cross-shell tests (see #551).

This fixes #565.

The only reason that works is a bug in the argument-validation code, which my urchin PR fixes.

However, my urchin PR requires that options come _before_ non-option arguments, so your variant would break.

Do you feel that urchin should support options even _after_ the (one and only) non-option argument?