The new algorithm improves efficiency with iteration in place of
recursion, and in a number of other ways:

- Avoid searches into dead-end branches. The traversal of all branches
was done because of the overlapping prefix case -- "foo" and "foobar"
both in the set. Now we just search the tree for a match, but before
descending into the next branch, check if there are other branches at
which the current prefix matches a terminating node.

- Only do string comparisons when we hit a terminating node.

- Mark terminating nodes with a flag in the tree, so that we don't go
looking for the null byte in the strings table during the search.

While we're here, rename the flag for the nibble search as hinib --
non-zero if and only if we inspect the most significant nibble at that
node. Also remove some dead code from QP_Insert().