korad-kaxxxxp: add support for forced "detection" of a given model

Korad PSU models are rather popular. But the successful operation of
currently unsupported model names or firmware versions is hard to verify
by users, because building the library from locally modified sources is
involved.

Introduce support for the "force_detect=" scan option. Warning messages
contain how the device identifies itself. Optional user specs can force
the assignment of the driver to the unsupported model. Which results in
reports that include the identification details as well as the successful
use of the device.

  $ sigrok-cli -d korad-kaxxxxp:conn=...:force_detect=KORADKA3005PV2.0 --show