chromium-bsu_disc*.sh: use stage 3 by default

It's better if the user can directly run the script and sees
something. In step 4 that's not the case if the unique code address
doesn't match. The relevant code address is easy to find as it has
the shortest backtrace. The backtrace is capped at 10 code
pointers.
ugtrain: support discovery stage 0

We can use the following option, now: -D 0.
This way libmemdisc just returns the stack end and the heap start.
libmemdisc: use heap_start as local variable