rdtech-dps: research to reduce serial comm transfer volume again

rdtech-dps: research to reduce serial comm transfer volume again

The DPS devices default to a rather low UART communication bitrate. The
generous retrieval of more Modbus registers than strictly necessary for
the periodic acquisition of analog channel values and state tracking may
or may not be an issue.

Instrument all call sites in api.c to the device state gathering calls
such that the protocol.c side transparently can reduce the transfer
volume depending on which level of detail the caller may expect given
its current context. Under optimal circumstances this might cut the
traffic in half, which DPS devices may benefit from.

This commit adds the infrastructure to potentially reduce transfer
volume, but does not change behaviour. The "config" reasons may need
further partitioning if Modbus register access really turns out to be
a severe bottleneck. The API lends itself to that extension easily. The
approach which gets prepared in this commit needs to get runtime tested
by those who got access to the hardware.