Fixed soft limits when the origin is in the middle.

Fixed soft limits when the origin is in the middle.
HOME_POS is now always where the endstop is and can be outside the limits.
The limits are now defined by MIN_POS and MAX_POS rather than HOME_POS and MAX_LENGTH.
The Z is axis now homed first if direction is away from the bed.

Saguinololu limit pins change from MIN to MAX according to the homing direction.