Move down to a safe height after G28 for DELTAs.

- Especially in `gcode_T`
- In `update_software_endstops`
- Some tweaks to observe

Don't deploy/stow the probe if z-min-endstop is used for homing.
Don't apply `zprobe_zoffset` if probe was not used.

Adjust wait_for_cooling slope
and drop mintemp for cooling.

See
https://github.com/MarlinFirmware/Marlin/pull/4169#issuecomment-231544532

when no known axis needs to be homed.

Most changes are only caused from altering the indentation.
```
if (axis == X_AXIS ? HOMEAXIS_DO(X) : axis == Y_AXIS ? HOMEAXIS_DO(Y) : axis == Z_AXIS ? HOMEAXIS_DO(Z) : 0) {
  ...
}

to

if (!(axis == X_AXIS ? HOMEAXIS_DO(X) : axis == Y_AXIS ? HOMEAXIS_DO(Y) : axis == Z_AXIS ? HOMEAXIS_DO(Z) : 0)) return;
...

```

For moving away from the endstops we do not have to disable them.
They are not tested anyway.

Use logic in Z_SAFE_HOMING

From
```
if (home_all_axis || homeZ) {
  if (home_all_axis) {
  ...
  home z
  }
  else if (homeZ) { // Don't need to Home Z twice
  home z
  }
}

```
to
```
if (home_all_axis || homeZ) {
  if (home_all_axis) {
  ...
  }
  home z
}
```

Always define MIN_Z_HEIGHT_FOR_HOMING even if 0
Always make a potential rise to `home_offset[Z_AXIS]` possible in G28.
Get rid of some very ugly constructs in MBL (ultralcd.cpp).

Differentiating made sense here when we could save a dozen lines of code,
but not for one line.

Since we do the actual homing now in the 'normal' routines `quick_home_xy()`
can be simplified to a relative simple diagonal move.

Add an emergency-command parser to MarlinSerial's RX interrupt.

The parser tries to find and execute M108,M112,M410 before the commands disappear in the RX-buffer.

To avoid false positives for M117, comments and commands followed by filenames (M23, M28, M30, M32, M33) are filtered.

This enables Marlin to receive and react on the Emergency command at all times - regardless of whether the buffers are full or not. It remains to convince hosts to send the commands. To inform the hosts about the new feature a new entry in the M115-report was made. "`EMERGENCY_CODES:M112,M108,M410;`".

The parser is fast. It only ever needs two switch decisions and one assignment of the new state for every character.

One problem remains. If the host has sent an incomplete line before sending an emergency command the emergency command could be omitted when the parser is in `state_IGNORE`.
In that case the host should send "\ncommand\n"

Also introduces M108 to break the waiting for the heaters in M109, M190 and M303.

Rename `cancel_heatup` to `wait_for_heatup` to better see the purpose.

Move and extend axis_unhomed test to deploy/stow_z_probe().
Move and extend position store/restore to deploy/stow_z_probe().

Now all kinds of probes can use the 'E' parameter in G29/M48.
Allen key probes can be used now for grid and 3-point levelling.

Deploying the Allen Key probe uses big moves in z direction.
Too dangerous for an unhomed z-axis.
Throw a compile time error when we try to configure an
Allen Key probe homing to z-min and having no other z-min-endstop.