OK, I'm slogging through figuring stuff out, and thus far, I've determined
  that yes, gif_output and in_gif_output get called repeatedly for
  a single packet.  I'd *like* to force __HAVE_GENERIC_SOFT_INTERRUPTS,
  and I tried #undef-ing it in sys/arch/i386/i386/vector.S and
  sys/arch/i386/include/types.h, but then I get a compile-time error:

Rather than try to turn that all off, I would change if_gif.c to
require both that and e.g. GIF_USE_SOFTINTR, so that the system still
has the soft interrupt _mechanism_, but that gif_output doesn't try to
_use_ it.  This may well blow up in some way I don't grok, though.