Add new option for Z_SAFE_HOMING_FIXED_POINT

Before any Z-home, including a G28 Z0, this will first home in X & Y and
then move to the z safe-homing X & Y point.

For printers where Z-homing is only safe at very specific X & Y points
this prevents extruder/bed crashes during an accidental G28 Z0.

It also disables G30.