.Dd and .TH macros (after loading the right sets of macros). Both were
being called with a different number of macros than they were actually
called with, and that was screwing up Dd's processing of its arguments.
This fix is groff-specific. It's apparently possible to do this is a
non-groff-specific way, but relatively hard, and my roff skills just aren't
up to it (and I have no intention of changing that 8-). I've marked the
lines that are groff-specific, at least.
