Change interactive-p to called-interactively-p.

The first was obsoleted in Emacs 23.2; this kind of sets a floor on
what version of Emacs I'm going to support with this.

See https://github.com/saintaardvark/twiki-org/issues/3.
Malformed function.  See https://github.com/saintaardvark/twiki-org/issues/2.