emacs: configure comint-mode

parent mode for several shells, including M-x shell and M-x run-python.

make the prompt text read-only. unbind "C-<up>" and "C-<down>" from
cycling input history. this means they can still be used for windmove-up
and windmove-down. and "M-p" and "M-n" can be used instead to cycle
through input history anyway.