(sctstr, shpstr, lndstr): Use `short' instead of `unsigned short' for

(sctstr, shpstr, lndstr): Use `short' instead of `unsigned short' for
item storage.  Rationale: Permitted values are 0..M, where M depends
on the container.  The largest M is ITEM_MAX (9999).  Benign
overflow/underflow occurs at those limits.  Catastrophic
overflow/underflow occurs at the limits of the underlying data type.
For `unsigned short', any underflow is catastrophic.  For `short',
benign undeflow happens long before catastrophic underflow.  Moreover,
unsigned arithmetic tends to trip up unwary programmers.