[LinuxCNC/linuxcnc PR#551] Apply minimum displacement checks consistently between canon and TP.

1) Change the minimum displacement calculation in PmCartLineInit to use
infinity-norm (i.e. max component) rather than the vector magnitude.
This is consistent with how Canon checks for minimum displacement
2) Apply the minimum displacement checks in canon with a threshold based
on user units, so that the cutoffs are the same for both the TP and
canon.

To be clear, this is an ugly way to fix the issue. The TP has to repeat
the work that canon does, and do so the exact same way to avoid weird
edge cases. The long-term fix needs to be more robust, perhaps with an
API change to enforce canon’s choice of linear / angular or pure-angular
move.