Few DMM drivers already did it. This commit adjusts the remaining DMM
drivers, to set the "DC" flag for measurements in diode mode.
This fixes bug #144.
Although I don't have the hardware to test, the nature of the change and
the arrangement of driver code suggests it's good. When a meter already
communicated the "DC" status, the change does nothing and won't harm.
The change ensures "DC" is flagged for those meters which previously
didn't, which is desirable.