Contains some fixes to properly handle unterminated functions, as well as
unknown functions.  In both cases we replace with an empty string.  For an
unterminated function, this is to the end of the line.  For an unknown
function, we parse it correctly, we just can't call it.

Still trying to figure out the unresolved_specials bit.  There has to be
a more sensible way to do this.  I will think on it later.
Nearly complete builtin support.  The builtins that process their own
arguments to prevent early execution now need to be extended to
handle unresolved symbols.