New Feature: Z_DUAL_ENDSTOPS

Z_DUAL_ENDSTOPS is a feature to enable the use of 2 endstops for both Z
steppers - Let's call them Z stepper and Z2 stepper.
That way the machine is capable to align the bed during home, since both
Z steppers are homed.
There is also an implementation of M666 (software endstops adjustment)
to this feature.
After Z homing, this adjustment is applied to just one of the steppers
in order to align the bed.
One just need to home the Z axis and measure the distance difference
between both Z axis and apply the math: Z adjust = Z - Z2.
If the Z stepper axis is closer to the bed, the measure Z > Z2 (yes, it
is.. think about it) and the Z adjust would be positive.
Play a little bit with small adjustments (0.5mm) and check the
behaviour.
The M119 (endstops report) will start reporting the Z2 Endstop as well.

Marlin/stepper.h

@@ -97,6 +97,12 @@ void digipot_current(uint8_t driver, int current);
 void microstep_init();
 void microstep_readings();
 
+#ifdef Z_DUAL_ENDSTOPS
+  void In_Homing_Process(bool state);
+  void Lock_z_motor(bool state);
+  void Lock_z2_motor(bool state);
+#endif
+
 #ifdef BABYSTEPPING
   void babystep(const uint8_t axis,const bool direction); // perform a short step with a single stepper motor, outside of any convention
 #endif