Enhance INTERNET->LAN rates

To achieve a target latency of 50ms (1/20th of a second) it is important
to take preventative action before the target is reached. This patch
changes the burst rates to half the target (25ms, or 1/40th a second)
and sets the cburst to the size of a typical packet (or "average packet"
as the `man tc-htb` calls it).

Experiment has shown that these changes keep ping latency to ~80ms when
the DOWNLINK is completely congested. (Ping is ~20ms when DOWNLINK is
otherwise idle). While this is not quite the target of 50ms, it is "good
enough".
Update UPFACTOR and DOWNFACTOR based on new experiments.

It would seem that the BRAS profile reported by Plusnet is no longer
accurate. I suspect that DOWNFACTOR is now next-to-useless on my
connection - speed tests (without shaping) show that the maximum
achievable rate is ~2.41 Mbps of a sync rate of 3.51 Mbps. Rather than
rework the script, for now I'll just fudge the DOWNFACTOR, since my sync
rate has been stable for months.