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/Marlin.h

@@ -242,6 +242,8 @@ extern float home_offset[3];
   extern float delta_diagonal_rod;
   extern float delta_segments_per_second;
   void recalc_delta_settings(float radius, float diagonal_rod);
+#elif defined(Z_DUAL_ENDSTOPS)
+extern float z_endstop_adj;
 #endif
 #ifdef SCARA
   extern float axis_scaling[3];  // Build size scaling