Fixes #7250

Addressing #7227

Sends pre-defined string as action command when printer is kill.

Probing with the effector in the printing area, but an eccentric probe (e.g. allen key) outside it but still touching the bed gives meaninfull information for calibration. Since calibration is most accurate when probing as close to the towers as possible the testing was way to restrictive hence this fix.

Allows the user to send text to the serial console in order to
communicate with a host - sending debuging information or action
commands, for example.  Text must begin with '//' and this is added if
it is not already present at the beginning of the string.

Can't compile with PINS_DEBUGING enabled on Atmega1284p because it tries
to enumerate pins 46 and 47 (which don't exist) because `#ifdef
AVR_AT90USB1286_FAMILY ` doesn't work as expected.

Addressing #6992 and #5851

Use the raw position for mesh measurement. Otherwise the `M206` and
`M92` Z offsets will get canceled out by bed leveling. The downside is
`G29` will not compensate for a poorly set small `M206` fudge value.

To elaborate on this issue, imagine you are probing with a Z home
offset of -0.1, meaning when Z homes, -0.1 is the current position,
implying the ideal bed zero for the nozzle is 0.1mm higher than the Z
endstop. Ordinarily when printing, Z would raise 0.1mm higher.

What happens when we probe is that all points are measured with that
-0.1 included. So when bed leveling is enabled the `M206 Z` offset gets
exactly canceled out by the bed readings.

* Add Z Fade Height to M420 Output

Echo the z-fade height when M420 is called, if it is enabled.

* Fix Whitespace

* Fix Compiler complaint

* Missing semi-colon

Addressing #7057

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Changed to conditional compile for AT90USB issue