rdtech-dps: layer separation between API and protocol, style nits

The existing power supply driver for Riden DPS/DPH devices unfortunately
duplicated intimate details of modbus communication and register layout
including interpretation of raw register values across several source
files. Do separate the physical transport and the register layout and
register fields interpretation layers. This brings the driver in line
with the usual api.c and protocol.c arrangement of responsibilities, and
prepares the future addition of other similar devices.

Address a few style nits while we are here. Include <config.h> in all
source files, and alpha-sort include directives. Address minor data type
nits. Reduce indentation in code paths. Propagate more error codes.
Update comments which lagged behind the code, adjust grammer (eliminate
"actual" false friends). Eliminate a few magic numbers and redundancies,
switch to the more recent endianess aware byte stream reader API where
beneficial, to eliminate redundant offset math.

Other nits seem to have gone unnoticed before: The indices of all sigrok
channels were identical (all zero). Signed print formats were used for
unsigned data.

This implementation compiles, but wasn't runtime tested due to lack of
hardware. A rough estimate of transfer volume and transport throughput
suggests that the 10ms cycle time no longer can be kept, may end up
around 25ms (40 cycles per second). This can get avoided by adding more
code to tell the configuration requests, the acquisition start, and the
measurements grabbing during acquisition apart. It's assumed though that
the 10ms glib poll interval did not translate to such fast measurement
gathering. The firmware may typically provide data at lower rates anyway.

The generic source code may look overly complicated or redundant at
first glance, but compilers' generated code will greatly get optimized.
Simplified maintenance because of reduced cognitive load is considered
more important.