kingst-la2016: rework acquisition limits, improve CLI use

kingst-la2016: rework acquisition limits, improve CLI use

Remove the rather arbitrary previous limits of 5MSa at 100MSa/s and 5%
capture ratio. Start with the highest available samplerate (per model)
and without any samples count limit. Use 50% capture ratio which is as
arbitrary a choice as the previous value but matches what scopes do.
Drop the lower samples count limit. (An arbitrary check encoded in the
library refuses to set a value of 0, so starting with 0 is the only way
of starting unlimited and being able to optionally specify a limit.)

This results in improved CLI use out of the box. Either of --samples or
--time works as expected, and both can be used in combination. The GUI
starts from a samples count limit by default that is consistent with
other devices. Users should be happy.

Accept any samplerate spec which is covered by hardware constraints.
Keep user provided specs unmodified for later reference, map these to
hardware register values which get forwarded to the device in a best
effort manner.

Make too high sample count limits non-fatal, just cap at 10GSa length.
The device's hardware compression which is affected by input signal
patterns already kept resulting in potentially shorter captures than
configured. There is no surprise here for users either. Use the model
dependent sample memory capacity to derive maximum pre-trigger memory
sizes, which eliminates another arbitrary magic number in the driver.