Don't use current-prefix-arg outside (interactive).
Fixes #28.
In detail:
* Simplify macro defp4cmd using arglist and backquote skill.
* New macro defp4cmd* handles common case where default arguments need to be evaluated both inside (interactive) to supply a default command-line if the user typed a prefix argument, and outside, to supply default arguments if no interactive arguments were supplied.
* Replace most defp4cmd using current-prefix-arg with defp4cmd*.
* In remaining cases, move current-prefix-arg inside (interactive).