Moved checking for car clicks into a function.

This maybe should be in a separate cpp file with prototype and all.
Still not convinced that it doesn't introduce some bug.  Next up,
add a pause button.
Added other instance of check_distances.

Now the function makes sense.  I forgot that I was doing it twice
depending on if we were paused or not.
Can now toggle pause with spacebar

Possibly I want this to be the only way to unpause.  Is it annoying
to have to unpause after moving a unit, or is it more annoying to
have to repause with space after issuing every command?