Ensure the probe offset will always be included when homing Z.

- Prevent `Z_SAFE_HOMING` from homing Z twice in some cases
- Allow `G28` with XYZ values of 0 to explicitly set the position
- Don’t add `home_offset` when setting XYZ explicitly in `G28`
- Add `code_has_value` function to check for the presence of a numeric
value (could just test for space or nul to allow other types of values)

- It’s inline here, but could be a macro and duplicated for
`planner.cpp`

- Use `line_to_current` in place of `plan_buffer_line`

Resolved some warnings in Marlin_main.cpp regarding compares of signed and unsigned integers by replacing 'int' or 'uint16_t' with 'uint8_t'.

and cleaned typo.

- Address Issue #1682 at hash #issuecomment-89756858

Should be `retracting` and not `retract`

- Document `manage_inactivity` function
- Allow `EXTRUDER_RUNOUT_PREVENT` to work with all extruders
- Use faster `memcpy` for copying coordinates

max_feedrate is in mm/s. line_to_destination needs a feedrate in mm/min because there is feedrate/60.

related to current_position[Z_AXIS] and retract_zlift.

'+ =' -> '+='

Fix for #1786

- Add comments documenting `thermal_runaway_protection`
- Add an enum for the thermal runaway states
- Add macros for temperature helper functions
- Fix a glitch with the z probe sled in homeaxis

* Use "G29 S3 Xn Yn Zn.nn" to modify bad probed point manually
* Changed manual Z steps from 0.05 to 0.025 and made brought it to Configuration.h

Updated documentation in Configuration.h.
Cleaned up and commented some code relating to Z_PROBE_ENDSTOP.
Separated Z_MIN_ENDSTOP and Z_PROBE_ENDSTOP completely.