Replaced !iscntrl() with isprint().

Replaced !iscntrl() with isprint().

To address Ralph's FIXME comment.  One example where a byte would
have been printed with !iscntrl() is 0x24 is ISO 646 BASIC (Inv),
which is undefined.  Also, added test cases to cover most of
get_param_value().  Also, replaced a couple of other FIXME's with
code comments.