[Fractdev] atan2_bf() problem

Tim,

It appears that atan2_bf() in bigflt.c isn't working correctly. This
affects cmplxlog_bf() and thus ComplexPower_bf() in fractalb.c. This
can be seen by starting up the manzpower fractal type and then switching
to arbitrary precision math using debug=3200.

I found this while I was coding the dividebrot5 fractal. In the case of
the dividebrot5 fractal, Fractint locks up. With the manzpower fractal,
an image is generated, but not the correct one.

Jonathan