motion: introduce motion.ferror-mode

by default (motion.ferror-mode == 0), ferror is computed based on the
last commanded position, which is problematic with velocity-mode drives

if motion.ferror-mode == 1, ferror is calculated based on the new commanded
position, and following error checks are postponed until after the new
commanded position is calculated.

see also: http://www.linuxcnc.org/index.php/english/forum/search?q=servo+tuning+advice&childforums=1