Use Control-D for quit

On linux / unix based systems, control-D inserts an EOF which should be
read as quitting.

Note: There was some code that I deleted here.  I think it might have used control-d as a second delete key, but this seems like the wrong thing to do - also, for an inexperienced linux user, doing

    fsharpi
    > <control-d>

will result in a pretty nasty crash at the moment which is probably not a good experience