More explicit formula for DDS residual
Get the timer dithering logic *just* right, including 2ppt Rb res
Print IP address to console even if static
Add fudge to NTP RX and TX times

The times from the old board seem to be pretty close (indicating that
most of the time is spent in the W5100, and in SPI for the TX). This
isn't closely vetted, but seems to be ~100us accurate.