trajectory planner: fix  F-word limit in position sync, enable blending by default

this are two commits from https://github.com/robEllenberg/linuxcnc-mirror/commits/circular-blend-arc-rc4

the F-word limit fix is an obvious plus

the other one enables the blending feature by default, which I think now is stable - it is equivalent to specifying

[TRAJ]
ARC_BLEND_ENABLE=1