make extra error code for sudoku, change tests

now grey output of the testing-function means, that the sudoku might
just not be finished.  Red means, that it was finished wrong.
add more comments
extend comments
add function to check visibility of two fields
use f_visible in check_chain_link
make functions static
replace pointless function by another function that does something slightly different
call build_golden_chain-stub
move build_golden_chain to the right place
start writing a function that builds the golden chain
move acc to desktop
rewrite stub for find_intersection
allocate memory
rename find_intersection to build_intersection
set intersections for almost every case
set section for every case
fix typos
fix pointer
i guess malloc is the wrong approach here
change section to desktop.intersection