New option for Z homing from only 1 point

New option `Z_SAFE_HOMING_FIXED_POINT`

Before any type of Z-home, including a `G28 Z0`, this will first home in X and Y and then move to the Z safe-homing point.

On some printer hardware, Z-homing is only safe at very specific (x,y) points.  So this prevents extruder/bed crashes during an accidental `G28 Z0` from unsafe points.

It also disables `G30` for the same reason.


Feedback welcome.  Would be glad to share info about hardware that needs this.  Also, I wasn't sure whether or not to put the error message for `G30` in `language.h`