mount_nexstaraux: fix southern hemisphere rotation directions

[keithbe]

When a Celestron Nexstar mount is aligned to the southern celestial pole on a wedge, tracking functions properly but a GOTO command turns the telescope in the wrong direction for both RA (az) and DEC (alt) axes. For southern EQ alignment the Celestron HC writes negated values to the motor controllers, so this change does the same. The guider code does not need to be changed because the "calibration" phase examines movement of the starfield with test offsets to work out signs and coefficients. This accounts for the reversed motor controller directions south of the equator.

This pull request contains two changes:

1. negates RA/DEC written to and read from motor controllers when located in southern hemisphere (fixing GOTO problem in southern hemisphere)
2. returns DEC in the range -90 to +90 degrees rather than 270->360->90 (avoids complaints in indigo logs)