Include all the necessary pieces to get the final Z position after
probing the bed, and add commentary about other possible changes.

Just set up the pin. Don't move to a random position.

Simplify servo::move()
* servo::move() does not need the pin parameter - The pin is set during servo_init() with attach().
* servo::move() does not need a return value.

SERVO_LEVELING is the wrong condition to deactivate the servos.

Remove some temporary (Servo *) variables.
SanityCheck for the servo indexes.

Updates to `set_homing_bump_feedrate`:
- Move the string into Program Memory, reduce length by 31 bytes
- Use an auto to get the divisor, adjust it on error
- Set feedrate once, at the end

- Change suggested at
https://github.com/MarlinFirmware/Marlin/issues/2482#issuecomment-123553
266

The compiler may be able to optimize if it knows an array won’t be
changing.

`gcode_M84` only needs to call `finishAndDisableSteppers`, which calls
all the other functions.

- Documentation solution for #2476

Assuming #2470 fixed up the main issue.

- Update prepare_move_delta to take a target argument
- Add Delta support to plan_arc

Patch servos code for move