Fixed a "bug" in debug on/off: typing debug string where any string!=on would turn debugging off. I decided it would be better
if we only allow string==on or string==off in the shell. Otherwise, print an error. Added debug statements to setprompt and
quit. Added debug info to chdir and added newline char to error message about changing directory.
Merge https://github.com/cszh222/ioshell