Disable keyboard input in "split" mode

Keyboard input does not seem useful in split mode, as the output is
probably meant to be parsed by another program.  Users can press ^C anyway
if they want to interrupt mtr.

It was helplessly broken anyway: no key press was actually taken into
account (see bug #30 on Github), *except* the "Enter" key, which would
cause an infinite loop by calling select() repeatedly.