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/language.h

@@ -128,6 +128,7 @@
 #define MSG_Y_MAX                           "y_max: "
 #define MSG_Z_MIN                           "z_min: "
 #define MSG_Z_MAX                           "z_max: "
+#define MSG_Z2_MAX                          "z2_max: "
 #define MSG_M119_REPORT                     "Reporting endstop status"
 #define MSG_ENDSTOP_HIT                     "TRIGGERED"
 #define MSG_ENDSTOP_OPEN                    "open"