Introduce an `UNLESS` command

[ecpeterson]

Many of the command definitions start with an (unless aborting? …) wrapper around their body. It would be preferable, I think, for this clause to be part of the procedure definition rather than the command definition. One could imagine a command along the lines of (:UNLESS slot contents) which pushes contents onto the command stack when slot evaluates to an affirmative.

This might be slightly off: it's often possible that the aborting? flag is set mid-procedure, which bypasses several commands / unwinds the continuation stack a fixed amount. In this sense, what we want is more like a condition handler, or a goto.