scpi: Log responses received by sr_scpi_get_string().
rigol-ds: Use common rigol_ds_config_set() function throughout.
rigol-ds: Overhaul VS5000 and DS1000 support.

This change moves the handling of series differences out to the points in the
code where they actually matter, unifying the overall structure of the code.

It also adds new VS5000/DS1000 series equivalents for commands that were
previously only implemented on the later models.

After this change, trigger waiting and the 'Memory' data source are supported
on the VS5000/DS1000 series.