testo: Initial driver implementation.

This adds support for the Testo 435-4, with differential pressure
measurement built in, and probes for wind speed and temperature/humidity
at 293 kelvin.

Support for other probe types will have to be added by people with
access to those probes.

Models other than the 435-4 may well work unchanged, but this is
difficult to predict. Most likely new unit types will need to be added,
and possibly the protocol handling may need to be more flexible and
model-dependent to cope with 5-byte values and other minor changes
in the protocol.