fix node control-handle locking to conform to documentation
This patch fixes the behaviour of control-handle locking during conversion to smooth and symmetric path nodes, to conform to what is described in the keyboard reference and the manual.
https://inkscape.org/en/doc/keys092.html#idm2176
http://tavmjong.free.fr/INKSCAPE/MANUAL/html/Paths-Editing.html
See https://bugs.launchpad.net/inkscape/+bug/1780775 for complete details.