Revise SCPI read API to allow backend-independent data handling.
Skip analog probes in logic-only output formats.
Add config keys for setting the number of analog probes.

This is primarily of use in the demo driver, but this patch also
takes in the logic probe setting config key used only by the
session driver so far.
demo: User-configurable number of probes.

The sigrok and incremental patterns repeat every 8 probes, shifted by
one probe.
demo: Analog probe support.
scpi: Strip trailing newline from *IDN response if present.