add BLTouch-related messages in english and (rusty) french;
add missing endstops.h in ultralcd.cpp;
fix misc. compiler warnings;
fix lsf_reset - ZERO macro can't handle a pointer as it would only memset the size of the pointer, not the size of the entire struct

Also...   The memory corruption issue may be fixed.   The GCC compiler
was inlining static functions and this caused the G29() stack frame to
become much larger than the AVR could handle.

* relocated ubl state to config. store:
 * removed a number of ubl state variables and padding which were largely unused - saved 58 bytes of both SRAM and EEPROM;
 * modified ubl sanity_check - no longer checks removed state variables that were otherwise unused, where checking didn't seem to accomplish anything, ultimately;
 * removed pre_initialized state, saving 64 bytes of SRAM;
 * removed automatic saving of UBL state after UBL activation/deactivation;
* consolidated multiple GRID_MAX_POINTS_X/Y to 'Global Leveling' section of EEPROM;
* minor update to G29 Sx notes/instructions;
* renamed mesh load and save parameter to 'slot' from 'm' for clarity;

fix maximum mesh_index_ array size at 16 (15+1);

Pretty much...  The code is in place.  Still more work to do.    But it
has a lot of hooks and variables in other code, so commit and merge
before I pick up a million 'Conflicts'.

As it turns out...   an unsigned int can not go to less than 0,  so the
loop never terminates.

- Make all `unified_bed_leveling` data/methods static
- Move some UBL-related variables into the class
- Replace `map_[xy]_index_to_bed_location` with `mesh_index_to_[xy]pos`

UBL's maps are cleaner.   And with a 1 value, G29 O 1  will generate a
map suitable to be Cut & Pasted into Excel so a Suface Map can be
generated.

The wait_for_user change totally broke UBL.   But there is stuff wrong
now in the thermal code and/or LCD Panel code.