stmhal: Improve REPL control codes; improve pyboard.py script.

Improvements are:

2 ctrl-C's are now needed to truly kill running script on pyboard, so
make CDC interface allow multiple ctrl-C's through at once (ie sending
b'\x03\x03' to pyboard now counts as 2 ctrl-C's).

ctrl-C in friendly-repl can now stop multi-line input.

In raw-repl mode, use ctrl-D to indicate end of running script, and also
end of any error message.  Thus, output of raw-repl is always at least 2
ctrl-D's and it's much easier to parse.

pyboard.py is now a bit faster, handles exceptions from pyboard better
(prints them and exits with exit code 1), prints out the pyboard output
while the script is running (instead of waiting till the end), and
allows to follow the output of a previous script when run with no
arguments.