This is in advance of moving non-linear bed leveling to the planner
class.

Also improve the documentation of this and related options, and set
EXTRUDE_MINTEMP to 170 everywhere.

No need for `MANUAL_HOME_POSITIONS`. Just set each one as-needed to
override the automatic behavior.

Rather than include twice, which is tricky but may confuse

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).