hp-3457a: Implement AC, ACDC, and four-wire resistance modes

hp-3457a: Implement AC, ACDC, and four-wire resistance modes

The driver did not look at the mq_flags provided with the
SR_CONF_MEASURED_QUANTITY key, and it defaulted to DC measurements.
Use the second member of the tuple provided by the config key, which
represents the flags for the measurement, and set the instrument's
measurement mode accordingly.