Fix M100 Free Memory Checker

M100 had numerious changes and quit working.   Part of the problem is
the overloading of the SERIAL_PROTOCOL functions.   Also, some of the
address arithmatic was changed to use char *ptr and passing ptr into the
SERIAL_PROTOCOL functions caused them to try to print a string instead
of a number.     M100 is working again.   Let's keep it that way!

M100 has been expanded to now have a function  int
free_memory_is_corrupted()  that can be called from other code to see if
the free space is still contiguous.  It may make sense to add a flag to
control its verbose nature but right now, the extra chit chat is very
helpful to know int free_memory_is_corrupted()  is doing the right thing
and what it found at various points when it was called.     A 'Show &
Tell' is coming up with int free_memory_is_corrupted().