Ulink handling -- fixed performance issue (per-Refentry sets of

Ulink handling -- fixed performance issue (per-Refentry sets of
Ulinks are now assembled once per document and stored in a
global variable).

It now takes roughly on the order of 15 to 20 percent longer to
process a doc with links numbered than it does to process it with
links unnumbered. Which may sound like a lot, but with a doc that
has 100+ links, on my machine, that still amounts to less than a
one-second difference (only about 0.7 seconds).