chain.c: Use `short' for word and chain lengths, except for QP tries.

chain.c: Use `short' for word and chain lengths, except for QP tries.

The maximum word length is 64 bytes, and the chain length is bounded by
the word length.  We're sort-of forced into `size_t' for the word length
with QP tries, because `Tnextl' wants to store the key length through a
pointer.