Math: create a circular modulus function

Used to calculate equivalent angle from -180,180 from any angle
from -540 to infinity.
Stabilization: use the circular_mod function

Removes cryptic math from main code path.
Stabilization: use circular modulus for poi mode

This is in a separate commit so the other can be cherry-pick'd
to next.
Stabilization: use bound_sym function

Stabilization was redefining a local copy of the identical
function.  Now it uses the library version.