Make rc_configtest behave like rc_pre and rc_post; i.e. don't define a default

Make rc_configtest behave like rc_pre and rc_post; i.e. don't define a default
function (each rc.d script is supposed to define its own if wanted).
This way, we can filter out the "configtest" action depending on whether the
function exists or not.
Adapt documentation.

tweak/ok kn@