revert meaning of third, optional argument, and add an optional fourth

revert meaning of third, optional argument, and add an optional fourth
argument:

  setlistener(<property>, <func> [, <initial=0> [, <persistent=1>]])

This definition defaults to the old behavior. The fourth argument can
be set to zero, in which case the function is only called when the
node value has actually changed. It should really default to zero, but
then all callers would have to be reviewed, which is a bit too dangerous
"shortly" before a release.