This ought to address a number of the problems with the
recent "dummy td" update as well as some older ones:

    - Slims down the qh_append_tds() to remove two pairs
      of "should be duplicate" logic so that
        * qh_make() only creates and initializes;
        * qh_append_tds() calls it earlier;
        * always appends with dummy, no routine qh updates.

    - Reworked qh_completions() ... simpler, better.
       * two notable FIXMEs gone, and a bug related to
         how they interacted with scatterlist i/o
       * fixed bugs (including one oops) exposed by
         using dummies more.

Passes basic testing:  most 'usbtest' cases, usb2 hub
with keyboard and CF adapter, storage enumeration.
So it seems less troublesome, though it's still not
as happy as I've seen it.

However, "testusb -at12" (running 'write unlink' tests)
still fails for me, and usb-storage gets unhappy when
it decides (why?  and unsuccessfully) to reset high speed
devices.  I'm still chasing those problems, which seem
to come from higher up in the stack.