Bug 264

- config_get() is not guaranteed to have a non-NULL sdi, so sdi must be checked before use. Keys that don't require an sdi may be returned before sdi is checked -- so are valid regardless of sdi. Keys that do require sdi must be handled only after the check. If sdi is required but is NULL, SR_ERR_ARG must be returned.

- config_list() must be very particular in what it returns, depending on sdi and/or probe group:
 - SR_CONF_SCAN_SETTINGS is always valid, regardless of sdi or probe group.
 - if sdi is NULL, nothing except SR_CONF_SCAN_SETTINGS can be provided.
 - if probe_group is NULL, only the SR_CONF_DEVICE_OPTIONS that are not specific to a probe group must be returned. If a probe group is non-NULL, this key must return a different list. There must be no overlap between the lists: either a key requires a probe group or it doesn't.

- If a driver needs probe groups at all, i.e. it has settings which are particular to one (group of) probes but not another, the config_get/set/list() implementation for a key that requires a probe group must return SR_ERR_NA.
probe group options should look like this, in general: