Make knifetool more robust against crashes. Fix some hole problems.

Sometimes split would fail to make a face - make sure it doesn't
crash if that happens.
Chain finding code for holes would sometimes go back to same
vertex on the face, which causes problems, so fixe that.
Also, disallowed a cut that starts at a vertex or edge and
cycles back to that same point in a face - this would make
a face with a repeated vertex, which may not be handled by
the rest of the system properly.

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