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Fix for commit from Brecht:

2006/11/19 00:07:32 CET
Fix for bug #5250: inaccurate conversion between edit and pose mode bones.

Two very bad bugs:
- replacing atan() with atan2() should also remove the M_PI correction!
This is the equivalent:
angle=atan(x/y); if(y<0) angle+=M_PI;
angle= atan2(x, y);

- the new NormalizedVecAngle2() call was negating an input vector, causing
calling code to screw up. All arithb.c calls should not alter input.