sigrok plugin: Ensure consistent logging prefix.

sigrok plugin: Ensure consistent logging prefix.

* Make sure all logging is using the "sigrok plugin: " prefix.
* Improved logging when handling the "LogLevel" option.
* Renamed free_drvopts() to sigrok_free_drvopts(). All functions now use the
  "sigrok_" prefix.
* Don't abort parsing a "Device" block when encountering an incorrect number of
  arguments. The cf_util_* functions will complain about that.