fix regression from #251

remove thread_measure_stack_usage from kernel_internal.h again
thread_measure_stack_usage documentation

fix documentation - the return value is the opposite of what it said
add and improve comments
renamed space to space_free (the name is documentation as well)
thread_measure_stack_ usage -> free

Rename the function as its name suggests the opposite of what it does.
fix stack_size comment in ps.c
Merge pull request #518 from LudwigOrtmann/thread_measure_stack

Thread measure stack: usage => free